Updated remoting.xhr API to use promises.

Removed access to the native XHR object used by the API.

This is a larger change than one might expect for two reasons: First,
because the native XHR object only allows the response content to be
retrieved while the onreadystatechange handler is executing, and
second, because the unit test for dns_blackhole_checker.js relied on
synchronous semantics which cannot be duplicated with promises because
when a promise is resolved, its "then" handlers are not called until
the next event cycle.

Review URL: https://codereview.chromium.org/1003433002

Cr-Commit-Position: refs/heads/master@{#321608}

15 files changed, 541 insertions, 425 deletions

diff --git a/remoting/webapp/app_remoting/js/app_remoting.js b/remoting/webapp/app_remoting/js/app_remoting.js
index 82983eb..b3fd07c 100644
--- a/remoting/webapp/app_remoting/js/app_remoting.js
+++ b/remoting/webapp/app_remoting/js/app_remoting.js
@@ -105,11 +105,11 @@ remoting.AppRemoting.prototype.start = function(connector, token) {
   /** @type {remoting.AppRemoting} */
   var that = this;
 
-  /** @param {XMLHttpRequest} xhr */
-  var parseAppHostResponse = function(xhr) {
-    if (xhr.status == 200) {
+  /** @param {!remoting.Xhr.Response} xhrResponse */
+  var parseAppHostResponse = function(xhrResponse) {
+    if (xhrResponse.status == 200) {
       var response = /** @type {remoting.AppRemoting.AppHostResponse} */
-          (base.jsonParseSafe(xhr.responseText));
+          (base.jsonParseSafe(xhrResponse.getText()));
       if (response &&
           response.status &&
           response.status == 'done' &&
@@ -157,16 +157,16 @@ remoting.AppRemoting.prototype.start = function(connector, token) {
       // TODO(garykac) Start using remoting.Error.fromHttpStatus once it has
       // been updated to properly report 'unknown' errors (rather than
       // reporting them as AUTHENTICATION_FAILED).
-      if (xhr.status == 0) {
+      if (xhrResponse.status == 0) {
         that.handleError(new remoting.Error(
             remoting.Error.Tag.NETWORK_FAILURE));
-      } else if (xhr.status == 401) {
+      } else if (xhrResponse.status == 401) {
         that.handleError(new remoting.Error(
             remoting.Error.Tag.AUTHENTICATION_FAILED));
-      } else if (xhr.status == 403) {
+      } else if (xhrResponse.status == 403) {
         that.handleError(new remoting.Error(
             remoting.Error.Tag.APP_NOT_AUTHORIZED));
-      } else if (xhr.status == 502 || xhr.status == 503) {
+      } else if (xhrResponse.status == 502 || xhrResponse.status == 503) {
         that.handleError(new remoting.Error(
             remoting.Error.Tag.SERVICE_UNAVAILABLE));
       } else {
@@ -175,12 +175,11 @@ remoting.AppRemoting.prototype.start = function(connector, token) {
     }
   };
 
-  remoting.xhr.start({
+  new remoting.Xhr({
     method: 'POST',
     url: that.runApplicationUrl(),
-    onDone: parseAppHostResponse,
     oauthToken: token
-  });
+  }).start().then(parseAppHostResponse);
 };
 
 /**
diff --git a/remoting/webapp/app_remoting/js/feedback_consent.js b/remoting/webapp/app_remoting/js/feedback_consent.js
index f792376..1437d05 100644
--- a/remoting/webapp/app_remoting/js/feedback_consent.js
+++ b/remoting/webapp/app_remoting/js/feedback_consent.js
@@ -127,21 +127,19 @@ function onToken(token) {
           '/applications/' + remoting.settings.getAppRemotingApplicationId() +
           '/hosts/'  + hostId +
           '/reportIssue';
-      /** @param {XMLHttpRequest} xhr */
-      var onDone = function(xhr) {
-        if (xhr.status >= 200 && xhr.status < 300) {
+      var onDone = function(/** !remoting.Xhr.Response */ response) {
+        if (response.status >= 200 && response.status < 300) {
           getUserInfo();
         } else {
           showError();
         }
       };
-      remoting.xhr.start({
+      new remoting.Xhr({
         method: 'POST',
         url: uri,
-        onDone: onDone,
         jsonContent: body,
         oauthToken: token
-      });
+      }).start().then(onDone);
     } else {
       getUserInfo();
     }
diff --git a/remoting/webapp/crd/js/dns_blackhole_checker.js b/remoting/webapp/crd/js/dns_blackhole_checker.js
index 4be4ab8..95b342c 100644
--- a/remoting/webapp/crd/js/dns_blackhole_checker.js
+++ b/remoting/webapp/crd/js/dns_blackhole_checker.js
@@ -38,7 +38,7 @@ remoting.DnsBlackholeChecker = function(signalStrategy) {
   /** @private */
   this.blackholeState_ = BlackholeState.PENDING;
 
-  /** @private {?XMLHttpRequest} */
+  /** @private {?remoting.Xhr} */
   this.xhr_ = null;
 };
 
@@ -85,11 +85,11 @@ remoting.DnsBlackholeChecker.prototype.connect = function(server,
 
   this.signalStrategy_.connect(server, username, authToken);
 
-  this.xhr_ = remoting.xhr.start({
+  this.xhr_ = new remoting.Xhr({
     method: 'GET',
-    url: remoting.DnsBlackholeChecker.URL_TO_REQUEST_,
-    onDone: this.onHttpRequestDone_.bind(this)
+    url: remoting.DnsBlackholeChecker.URL_TO_REQUEST_
   });
+  this.xhr_.start().then(this.onHttpRequestDone_.bind(this));
 };
 
 remoting.DnsBlackholeChecker.prototype.getState = function() {
@@ -153,12 +153,12 @@ remoting.DnsBlackholeChecker.prototype.onWrappedSignalStrategyStateChanged_ =
 };
 
 /**
- * @param {XMLHttpRequest} xhr
+ * @param {!remoting.Xhr.Response} response
  * @private
  */
-remoting.DnsBlackholeChecker.prototype.onHttpRequestDone_ = function(xhr) {
+remoting.DnsBlackholeChecker.prototype.onHttpRequestDone_ = function(response) {
   this.xhr_ = null;
-  if (xhr.status >= 200 && xhr.status <= 299) {
+  if (response.status >= 200 && response.status <= 299) {
     console.log("DNS blackhole check succeeded.");
     this.blackholeState_ = BlackholeState.OPEN;
     if (this.signalStrategy_.getState() ==
@@ -166,14 +166,15 @@ remoting.DnsBlackholeChecker.prototype.onHttpRequestDone_ = function(xhr) {
       this.setState_(remoting.SignalStrategy.State.CONNECTED);
     }
   } else {
-    console.error("DNS blackhole check failed: " + xhr.status + " " +
-                  xhr.statusText + ". Response URL: " + xhr.responseURL +
-                  ". Response Text: " + xhr.responseText);
+    console.error("DNS blackhole check failed: " + response.status + " " +
+                  response.statusText + ". Response URL: " +
+                  response.url + ". Response Text: " +
+                  response.getText());
     this.blackholeState_ = BlackholeState.BLOCKED;
     base.dispose(this.signalStrategy_);
     this.setState_(remoting.SignalStrategy.State.FAILED);
   }
-}
+};
 
 /**
  * @param {remoting.SignalStrategy.State} newState
diff --git a/remoting/webapp/crd/js/dns_blackhole_checker_unittest.js b/remoting/webapp/crd/js/dns_blackhole_checker_unittest.js
index 9b0be49..e451fef 100644
--- a/remoting/webapp/crd/js/dns_blackhole_checker_unittest.js
+++ b/remoting/webapp/crd/js/dns_blackhole_checker_unittest.js
@@ -23,13 +23,15 @@ var checker = null;
 
 /** @type {remoting.MockSignalStrategy} */
 var signalStrategy = null;
-var fakeXhrs;
+
+/** @type {sinon.FakeXhr} */
+var fakeXhr = null;
 
 QUnit.module('dns_blackhole_checker', {
   beforeEach: function(assert) {
-    fakeXhrs = [];
     sinon.useFakeXMLHttpRequest().onCreate = function(xhr) {
-      fakeXhrs.push(xhr);
+      QUnit.equal(fakeXhr, null, 'exactly one XHR is issued');
+      fakeXhr = xhr;
     };
 
     onStateChange = sinon.spy();
@@ -46,9 +48,8 @@ QUnit.module('dns_blackhole_checker', {
     sinon.assert.calledWith(signalStrategy.connect, 'server', 'username',
                             'authToken');
 
-    assert.equal(fakeXhrs.length, 1, 'exactly one XHR is issued');
     assert.equal(
-        fakeXhrs[0].url, remoting.DnsBlackholeChecker.URL_TO_REQUEST_,
+        fakeXhr.url, remoting.DnsBlackholeChecker.URL_TO_REQUEST_,
         'the correct URL is requested');
   },
   afterEach: function() {
@@ -59,112 +60,136 @@ QUnit.module('dns_blackhole_checker', {
     onStateChange = null;
     onIncomingStanzaCallback = null;
     checker = null;
-  },
+    fakeXhr = null;
+  }
 });
 
 QUnit.test('success',
   function(assert) {
-    fakeXhrs[0].respond(200);
-    sinon.assert.notCalled(onStateChange);
-
-    [
-      remoting.SignalStrategy.State.CONNECTING,
-      remoting.SignalStrategy.State.HANDSHAKE,
-      remoting.SignalStrategy.State.CONNECTED
-    ].forEach(function(state) {
+    function checkState(state) {
       signalStrategy.setStateForTesting(state);
       sinon.assert.calledWith(onStateChange, state);
       assert.equal(checker.getState(), state);
+    }
+
+    return base.SpyPromise.run(function() {
+      fakeXhr.respond(200);
+    }).then(function() {
+      sinon.assert.notCalled(onStateChange);
+      checkState(remoting.SignalStrategy.State.CONNECTING);
+      checkState(remoting.SignalStrategy.State.HANDSHAKE);
+      checkState(remoting.SignalStrategy.State.CONNECTED);
     });
-  }
-);
+  });
 
 QUnit.test('http response after connected',
   function(assert) {
-    [
-      remoting.SignalStrategy.State.CONNECTING,
-      remoting.SignalStrategy.State.HANDSHAKE,
-    ].forEach(function(state) {
+    function checkState(state) {
       signalStrategy.setStateForTesting(state);
       sinon.assert.calledWith(onStateChange, state);
       assert.equal(checker.getState(), state);
-    });
+    }
+
+    checkState(remoting.SignalStrategy.State.CONNECTING);
+    checkState(remoting.SignalStrategy.State.HANDSHAKE);
     onStateChange.reset();
 
     // Verify that DnsBlackholeChecker stays in HANDSHAKE state even if the
     // signal strategy has connected.
-    signalStrategy.setStateForTesting(remoting.SignalStrategy.State.CONNECTED);
-    sinon.assert.notCalled(onStateChange);
+    return base.SpyPromise.run(function() {
+      signalStrategy.setStateForTesting(
+          remoting.SignalStrategy.State.CONNECTED);
+    }).then(function() {
+      sinon.assert.notCalled(onStateChange);
     assert.equal(checker.getState(), remoting.SignalStrategy.State.HANDSHAKE);
 
-    // Verify that DnsBlackholeChecker goes to CONNECTED state after the
-    // the HTTP request has succeeded.
-    fakeXhrs[0].respond(200);
-    sinon.assert.calledWith(onStateChange,
-                            remoting.SignalStrategy.State.CONNECTED);
-  }
-);
+      // Verify that DnsBlackholeChecker goes to CONNECTED state after the
+      // the HTTP request has succeeded.
+      return base.SpyPromise.run(function() {
+        fakeXhr.respond(200);
+      });
+    }).then(function() {
+      sinon.assert.calledWith(onStateChange,
+                              remoting.SignalStrategy.State.CONNECTED);
+    });
+  });
 
 QUnit.test('connect failed',
   function(assert) {
-    fakeXhrs[0].respond(200);
-    sinon.assert.notCalled(onStateChange);
-
-    [
-      remoting.SignalStrategy.State.CONNECTING,
-      remoting.SignalStrategy.State.FAILED
-    ].forEach(function(state) {
+    function checkState(state) {
       signalStrategy.setStateForTesting(state);
       sinon.assert.calledWith(onStateChange, state);
+    };
+
+    return base.SpyPromise.run(function() {
+      fakeXhr.respond(200);
+    }).then(function() {
+      sinon.assert.notCalled(onStateChange);
+      checkState(remoting.SignalStrategy.State.CONNECTING);
+      checkState(remoting.SignalStrategy.State.FAILED);
     });
-}
-);
+  });
 
 QUnit.test('blocked',
   function(assert) {
-    fakeXhrs[0].respond(400);
-    sinon.assert.calledWith(onStateChange,
-                            remoting.SignalStrategy.State.FAILED);
+    function checkState(state) {
     assert.equal(checker.getError().getTag(),
                  remoting.Error.Tag.NOT_AUTHORIZED);
-    onStateChange.reset();
-
-    [
-      remoting.SignalStrategy.State.CONNECTING,
-      remoting.SignalStrategy.State.HANDSHAKE,
-      remoting.SignalStrategy.State.CONNECTED
-    ].forEach(function(state) {
+      onStateChange.reset();
       signalStrategy.setStateForTesting(state);
       sinon.assert.notCalled(onStateChange);
-      assert.equal(checker.getState(), remoting.SignalStrategy.State.FAILED);
+      assert.equal(checker.getState(),
+          checker.getState(),
+          remoting.SignalStrategy.State.FAILED,
+          'checker state is still FAILED');
+    };
+
+    return base.SpyPromise.run(function() {
+      fakeXhr.respond(400);
+    }).then(function() {
+      sinon.assert.calledWith(
+          onStateChange, remoting.SignalStrategy.State.FAILED);
+      assert.equal(
+          checker.getError().getTag(),
+          remoting.Error.Tag.NOT_AUTHORIZED,
+          'checker error is NOT_AUTHORIZED');
+      checkState(remoting.SignalStrategy.State.CONNECTING);
+      checkState(remoting.SignalStrategy.State.HANDSHAKE);
+      checkState(remoting.SignalStrategy.State.FAILED);
     });
-  }
-);
+  });
 
 QUnit.test('blocked after connected',
   function(assert) {
-    [
-      remoting.SignalStrategy.State.CONNECTING,
-      remoting.SignalStrategy.State.HANDSHAKE,
-    ].forEach(function(state) {
+    function checkState(state) {
       signalStrategy.setStateForTesting(state);
       sinon.assert.calledWith(onStateChange, state);
       assert.equal(checker.getState(), state);
-    });
+    };
+
+    checkState(remoting.SignalStrategy.State.CONNECTING);
+    checkState(remoting.SignalStrategy.State.HANDSHAKE);
     onStateChange.reset();
 
-    // Verify that DnsBlackholeChecker stays in HANDSHAKE state even if the
-    // signal strategy has connected.
-    signalStrategy.setStateForTesting(remoting.SignalStrategy.State.CONNECTED);
-    sinon.assert.notCalled(onStateChange);
+    // Verify that DnsBlackholeChecker stays in HANDSHAKE state even
+    // if the signal strategy has connected.
+    return base.SpyPromise.run(function() {
+      signalStrategy.setStateForTesting(
+          remoting.SignalStrategy.State.CONNECTED);
+    }).then(function() {
+      sinon.assert.notCalled(onStateChange);
     assert.equal(checker.getState(), remoting.SignalStrategy.State.HANDSHAKE);
 
-    // Verify that DnsBlackholeChecker goes to FAILED state after it gets the
-    // blocked HTTP response.
-    fakeXhrs[0].respond(400);
-    sinon.assert.calledWith(onStateChange,
-                            remoting.SignalStrategy.State.FAILED);
+      // Verify that DnsBlackholeChecker goes to FAILED state after it
+      // gets the blocked HTTP response.
+      return base.SpyPromise.run(function() {
+        fakeXhr.respond(400);
+      });
+    }).then(function() {
+      sinon.assert.calledWith(onStateChange,
+                              remoting.SignalStrategy.State.FAILED);
     assert.ok(checker.getError().hasTag(remoting.Error.Tag.NOT_AUTHORIZED));
+    });
   }
 );
 
diff --git a/remoting/webapp/crd/js/host_controller.js b/remoting/webapp/crd/js/host_controller.js
index 61529f2..162cb2a 100644
--- a/remoting/webapp/crd/js/host_controller.js
+++ b/remoting/webapp/crd/js/host_controller.js
@@ -234,14 +234,14 @@ remoting.HostController.prototype.start = function(hostPin, consent, onDone,
    * @param {string} hostName
    * @param {string} publicKey
    * @param {string} privateKey
-   * @param {XMLHttpRequest} xhr
+   * @param {!remoting.Xhr.Response} response
    */
   function onRegistered(
-      hostName, publicKey, privateKey, xhr) {
-    var success = (xhr.status == 200);
+      hostName, publicKey, privateKey, response) {
+    var success = (response.status == 200);
 
     if (success) {
-      var result = base.jsonParseSafe(xhr.responseText);
+      var result = base.jsonParseSafe(response.getText());
       if ('data' in result && 'authorizationCode' in result['data']) {
         that.hostDaemonFacade_.getCredentialsFromAuthCode(
             result['data']['authorizationCode'],
@@ -260,8 +260,8 @@ remoting.HostController.prototype.start = function(hostPin, consent, onDone,
             });
       }
     } else {
-      console.log('Failed to register the host. Status: ' + xhr.status +
-                  ' response: ' + xhr.responseText);
+      console.log('Failed to register the host. Status: ' + response.status +
+                  ' response: ' + response.getText());
       onError(new remoting.Error(remoting.Error.Tag.REGISTRATION_FAILED));
     }
   }
@@ -281,16 +281,15 @@ remoting.HostController.prototype.start = function(hostPin, consent, onDone,
        publicKey: publicKey
     } };
 
-    remoting.xhr.start({
+    new remoting.Xhr({
       method: 'POST',
       url: remoting.settings.DIRECTORY_API_BASE_URL + '/@me/hosts',
       urlParams: {
         hostClientId: hostClientId
       },
-      onDone: onRegistered.bind(null, hostName, publicKey, privateKey),
       jsonContent: newHostDetails,
       oauthToken: oauthToken
-    });
+    }).start().then(onRegistered.bind(null, hostName, publicKey, privateKey));
   }
 
   /**
diff --git a/remoting/webapp/crd/js/host_list_api_impl.js b/remoting/webapp/crd/js/host_list_api_impl.js
index a4dbe08..52f9377 100644
--- a/remoting/webapp/crd/js/host_list_api_impl.js
+++ b/remoting/webapp/crd/js/host_list_api_impl.js
@@ -28,17 +28,16 @@ remoting.HostListApiImpl = function() {
  * @param {function(!remoting.Error):void} onError
  */
 remoting.HostListApiImpl.prototype.get = function(onDone, onError) {
-  /** @type {function(XMLHttpRequest):void} */
+  /** @type {function(!remoting.Xhr.Response):void} */
   var parseHostListResponse =
       this.parseHostListResponse_.bind(this, onDone, onError);
   /** @param {string} token */
   var onToken = function(token) {
-    remoting.xhr.start({
+    new remoting.Xhr({
       method: 'GET',
       url: remoting.settings.DIRECTORY_API_BASE_URL + '/@me/hosts',
-      onDone: parseHostListResponse,
       oauthToken: token
-    });
+    }).start().then(parseHostListResponse);
   };
   remoting.identity.getToken().then(onToken, remoting.Error.handler(onError));
 };
@@ -63,13 +62,12 @@ remoting.HostListApiImpl.prototype.put =
         'publicKey': hostPublicKey
       }
     };
-    remoting.xhr.start({
+    new remoting.Xhr({
       method: 'PUT',
       url: remoting.settings.DIRECTORY_API_BASE_URL + '/@me/hosts/' + hostId,
-      onDone: remoting.xhr.defaultResponse(onDone, onError),
       jsonContent: newHostDetails,
       oauthToken: token
-    });
+    }).start().then(remoting.Xhr.defaultResponse(onDone, onError));
   };
   remoting.identity.getToken().then(onToken, remoting.Error.handler(onError));
 };
@@ -84,13 +82,12 @@ remoting.HostListApiImpl.prototype.put =
 remoting.HostListApiImpl.prototype.remove = function(hostId, onDone, onError) {
   /** @param {string} token */
   var onToken = function(token) {
-    remoting.xhr.start({
+    new remoting.Xhr({
       method: 'DELETE',
       url: remoting.settings.DIRECTORY_API_BASE_URL + '/@me/hosts/' + hostId,
-      onDone: remoting.xhr.defaultResponse(onDone, onError,
-                                           [remoting.Error.Tag.NOT_FOUND]),
       oauthToken: token
-    });
+    }).start().then(remoting.Xhr.defaultResponse(
+        onDone, onError, [remoting.Error.Tag.NOT_FOUND]));
   };
   remoting.identity.getToken().then(onToken, remoting.Error.handler(onError));
 };
@@ -102,19 +99,19 @@ remoting.HostListApiImpl.prototype.remove = function(hostId, onDone, onError) {
  *
  * @param {function(Array<remoting.Host>):void} onDone
  * @param {function(!remoting.Error):void} onError
- * @param {XMLHttpRequest} xhr
+ * @param {!remoting.Xhr.Response} response
  * @private
  */
 remoting.HostListApiImpl.prototype.parseHostListResponse_ =
-    function(onDone, onError, xhr) {
-  if (xhr.status == 200) {
-    var response = /** @type {{data: {items: Array}}} */
-        (base.jsonParseSafe(xhr.responseText));
-    if (!response || !response.data) {
+    function(onDone, onError, response) {
+  if (response.status == 200) {
+    var obj = /** @type {{data: {items: Array}}} */
+        (base.jsonParseSafe(response.getText()));
+    if (!obj || !obj.data) {
       console.error('Invalid "hosts" response from server.');
       onError(remoting.Error.unexpected());
     } else {
-      var items = response.data.items || [];
+      var items = obj.data.items || [];
       var hosts = items.map(
         function(/** Object */ item) {
           var host = new remoting.Host();
@@ -134,7 +131,7 @@ remoting.HostListApiImpl.prototype.parseHostListResponse_ =
       onDone(hosts);
     }
   } else {
-    onError(remoting.Error.fromHttpStatus(xhr.status));
+    onError(remoting.Error.fromHttpStatus(response.status));
   }
 };
 
@@ -142,4 +139,3 @@ remoting.HostListApiImpl.prototype.parseHostListResponse_ =
 remoting.hostListApi = new remoting.HostListApiImpl();
 
 })();
-
diff --git a/remoting/webapp/crd/js/it2me_connect_flow.js b/remoting/webapp/crd/js/it2me_connect_flow.js
index 20f42c2..df85817 100644
--- a/remoting/webapp/crd/js/it2me_connect_flow.js
+++ b/remoting/webapp/crd/js/it2me_connect_flow.js
@@ -54,8 +54,8 @@ remoting.It2MeConnectFlow.prototype.connect_ = function(accessCode) {
     return remoting.identity.getToken();
   }).then(function(/** string */ token) {
     return that.getHostInfo_(token);
-  }).then(function(/** XMLHttpRequest */ xhr) {
-    return that.onHostInfo_(xhr);
+  }).then(function(/** !remoting.Xhr.Response */ response) {
+    return that.onHostInfo_(response);
   }).then(function(/** remoting.Host */ host) {
     that.sessionConnector_.connect(
         remoting.DesktopConnectedView.Mode.IT2ME,
@@ -88,33 +88,31 @@ remoting.It2MeConnectFlow.prototype.verifyAccessCode_ = function(accessCode) {
  * Continues an IT2Me connection once an access token has been obtained.
  *
  * @param {string} token An OAuth2 access token.
- * @return {Promise<XMLHttpRequest>}
+ * @return {Promise<!remoting.Xhr.Response>}
  * @private
  */
 remoting.It2MeConnectFlow.prototype.getHostInfo_ = function(token) {
   var that = this;
-  return new Promise(function(resolve) {
-    remoting.xhr.start({
-      method: 'GET',
-      url: remoting.settings.DIRECTORY_API_BASE_URL + '/support-hosts/' +
-           encodeURIComponent(that.hostId_),
-      onDone: resolve,
-      oauthToken: token
-    });
-  });
+  return new remoting.Xhr({
+    method: 'GET',
+    url: remoting.settings.DIRECTORY_API_BASE_URL + '/support-hosts/' +
+        encodeURIComponent(that.hostId_),
+    oauthToken: token
+  }).start();
 };
 
 /**
  * Continues an IT2Me connection once the host JID has been looked up.
  *
- * @param {XMLHttpRequest} xhr The server response to the support-hosts query.
+ * @param {!remoting.Xhr.Response} xhrResponse The server response to the
+ *     support-hosts query.
  * @return {!Promise<!remoting.Host>} Rejects on error.
  * @private
  */
-remoting.It2MeConnectFlow.prototype.onHostInfo_ = function(xhr) {
-  if (xhr.status == 200) {
+remoting.It2MeConnectFlow.prototype.onHostInfo_ = function(xhrResponse) {
+  if (xhrResponse.status == 200) {
     var response = /** @type {{data: {jabberId: string, publicKey: string}}} */
-        (base.jsonParseSafe(xhr.responseText));
+        (base.jsonParseSafe(xhrResponse.getText()));
     if (response && response.data &&
         response.data.jabberId && response.data.publicKey) {
       var host = new remoting.Host();
@@ -128,11 +126,12 @@ remoting.It2MeConnectFlow.prototype.onHostInfo_ = function(xhr) {
       return Promise.reject(remoting.Error.unexpected());
     }
   } else {
-    return Promise.reject(translateSupportHostsError(xhr.status));
+    return Promise.reject(translateSupportHostsError(xhrResponse.status));
   }
 };
 
 /**
+ * TODO(jrw): Replace with remoting.Error.fromHttpStatus.
  * @param {number} error An HTTP error code returned by the support-hosts
  *     endpoint.
  * @return {remoting.Error} The equivalent remoting.Error code.
diff --git a/remoting/webapp/crd/js/oauth2.js b/remoting/webapp/crd/js/oauth2.js
index d91da5d..37570b8 100644
--- a/remoting/webapp/crd/js/oauth2.js
+++ b/remoting/webapp/crd/js/oauth2.js
@@ -250,7 +250,7 @@ remoting.OAuth2.prototype.onTokens_ =
 remoting.OAuth2.prototype.getAuthorizationCode = function(onDone) {
   var xsrf_token = base.generateXsrfToken();
   var GET_CODE_URL = this.getOAuth2AuthEndpoint_() + '?' +
-    remoting.xhr.urlencodeParamHash({
+    remoting.Xhr.urlencodeParamHash({
           'client_id': this.getClientId_(),
           'redirect_uri': this.getRedirectUri_(),
           'scope': this.SCOPE_,
diff --git a/remoting/webapp/crd/js/oauth2_api_impl.js b/remoting/webapp/crd/js/oauth2_api_impl.js
index f8ed194..de5d9df 100644
--- a/remoting/webapp/crd/js/oauth2_api_impl.js
+++ b/remoting/webapp/crd/js/oauth2_api_impl.js
@@ -75,37 +75,36 @@ remoting.OAuth2ApiImpl.prototype.interpretXhrStatus_ =
  */
 remoting.OAuth2ApiImpl.prototype.refreshAccessToken = function(
     onDone, onError, clientId, clientSecret, refreshToken) {
-  /** @param {XMLHttpRequest} xhr */
-  var onResponse = function(xhr) {
-    if (xhr.status == 200) {
+  /** @param {!remoting.Xhr.Response} response */
+  var onResponse = function(response) {
+    if (response.status == 200) {
       try {
         // Don't use base.jsonParseSafe here unless you also include base.js,
         // otherwise this won't work from the OAuth trampoline.
         // TODO(jamiewalch): Fix this once we're no longer using the trampoline.
-        var tokens = JSON.parse(xhr.responseText);
+        var tokens = JSON.parse(response.getText());
         onDone(tokens['access_token'], tokens['expires_in']);
       } catch (/** @type {Error} */ err) {
         console.error('Invalid "token" response from server:', err);
         onError(remoting.Error.unexpected());
       }
     } else {
-      console.error('Failed to refresh token. Status: ' + xhr.status +
-                    ' response: ' + xhr.responseText);
-      onError(fromHttpStatus(xhr.status));
+      console.error('Failed to refresh token. Status: ' + response.status +
+                    ' response: ' + response.getText());
+      onError(remoting.Error.fromHttpStatus(response.status));
     }
   };
 
-  remoting.xhr.start({
+  new remoting.Xhr({
     method: 'POST',
     url: this.getOAuth2TokenEndpoint_(),
-    onDone: onResponse,
     formContent: {
       'client_id': clientId,
       'client_secret': clientSecret,
       'refresh_token': refreshToken,
       'grant_type': 'refresh_token'
     }
-  });
+  }).start().then(onResponse);
 };
 
 /**
@@ -124,14 +123,14 @@ remoting.OAuth2ApiImpl.prototype.refreshAccessToken = function(
  */
 remoting.OAuth2ApiImpl.prototype.exchangeCodeForTokens = function(
     onDone, onError, clientId, clientSecret, code, redirectUri) {
-  /** @param {XMLHttpRequest} xhr */
-  var onResponse = function(xhr) {
-    if (xhr.status == 200) {
+  /** @param {!remoting.Xhr.Response} response */
+  var onResponse = function(response) {
+    if (response.status == 200) {
       try {
         // Don't use base.jsonParseSafe here unless you also include base.js,
         // otherwise this won't work from the OAuth trampoline.
         // TODO(jamiewalch): Fix this once we're no longer using the trampoline.
-        var tokens = JSON.parse(xhr.responseText);
+        var tokens = JSON.parse(response.getText());
         onDone(tokens['refresh_token'],
                tokens['access_token'], tokens['expires_in']);
       } catch (/** @type {Error} */ err) {
@@ -139,16 +138,15 @@ remoting.OAuth2ApiImpl.prototype.exchangeCodeForTokens = function(
         onError(remoting.Error.unexpected());
       }
     } else {
-      console.error('Failed to exchange code for token. Status: ' + xhr.status +
-                    ' response: ' + xhr.responseText);
-      onError(fromHttpStatus(xhr.status));
+      console.error('Failed to exchange code for token. Status: ' +
+                    response.status + ' response: ' + response.getText());
+      onError(remoting.Error.fromHttpStatus(response.status));
     }
   };
 
-  remoting.xhr.start({
+  new remoting.Xhr({
     method: 'POST',
     url: this.getOAuth2TokenEndpoint_(),
-    onDone: onResponse,
     formContent: {
       'client_id': clientId,
       'client_secret': clientSecret,
@@ -156,7 +154,7 @@ remoting.OAuth2ApiImpl.prototype.exchangeCodeForTokens = function(
       'code': code,
       'grant_type': 'authorization_code'
     }
-  });
+  }).start().then(onResponse);
 };
 
 /**
@@ -170,28 +168,27 @@ remoting.OAuth2ApiImpl.prototype.exchangeCodeForTokens = function(
  * @return {void} Nothing.
  */
 remoting.OAuth2ApiImpl.prototype.getEmail = function(onDone, onError, token) {
-  /** @param {XMLHttpRequest} xhr */
-  var onResponse = function(xhr) {
-    if (xhr.status == 200) {
+  /** @param {!remoting.Xhr.Response} response */
+  var onResponse = function(response) {
+    if (response.status == 200) {
       try {
-        var result = JSON.parse(xhr.responseText);
+        var result = JSON.parse(response.getText());
         onDone(result['email']);
       } catch (/** @type {Error} */ err) {
         console.error('Invalid "userinfo" response from server:', err);
         onError(remoting.Error.unexpected());
       }
     } else {
-      console.error('Failed to get email. Status: ' + xhr.status +
-                    ' response: ' + xhr.responseText);
-      onError(fromHttpStatus(xhr.status));
+      console.error('Failed to get email. Status: ' + response.status +
+                    ' response: ' + response.getText());
+      onError(remoting.Error.fromHttpStatus(response.status));
     }
   };
-  remoting.xhr.start({
+  new remoting.Xhr({
     method: 'GET',
     url: this.getOAuth2ApiUserInfoEndpoint_(),
-    onDone: onResponse,
     oauthToken: token
-  });
+  }).start().then(onResponse);
 };
 
 /**
@@ -206,28 +203,27 @@ remoting.OAuth2ApiImpl.prototype.getEmail = function(onDone, onError, token) {
  */
 remoting.OAuth2ApiImpl.prototype.getUserInfo =
     function(onDone, onError, token) {
-  /** @param {XMLHttpRequest} xhr */
-  var onResponse = function(xhr) {
-    if (xhr.status == 200) {
+  /** @param {!remoting.Xhr.Response} response */
+  var onResponse = function(response) {
+    if (response.status == 200) {
       try {
-        var result = JSON.parse(xhr.responseText);
+        var result = JSON.parse(response.getText());
         onDone(result['email'], result['name']);
       } catch (/** @type {Error} */ err) {
         console.error('Invalid "userinfo" response from server:', err);
         onError(remoting.Error.unexpected());
       }
     } else {
-      console.error('Failed to get user info. Status: ' + xhr.status +
-                    ' response: ' + xhr.responseText);
-      onError(fromHttpStatus(xhr.status));
+      console.error('Failed to get user info. Status: ' + response.status +
+                    ' response: ' + response.getText());
+      onError(remoting.Error.fromHttpStatus(response.status));
     }
   };
-  remoting.xhr.start({
+  new remoting.Xhr({
     method: 'GET',
     url: this.getOAuth2ApiUserInfoEndpoint_(),
-    onDone: onResponse,
     oauthToken: token
-  });
+  }).start().then(onResponse);
 };
 
 /** @returns {!remoting.Error} */
diff --git a/remoting/webapp/crd/js/session_connector_impl.js b/remoting/webapp/crd/js/session_connector_impl.js
index 427d6a3..0d9ea5b 100644
--- a/remoting/webapp/crd/js/session_connector_impl.js
+++ b/remoting/webapp/crd/js/session_connector_impl.js
@@ -75,6 +75,9 @@ remoting.SessionConnectorImpl = function(clientContainer, onConnected, onError,
 remoting.SessionConnectorImpl.prototype.resetConnection_ = function() {
   this.closeSession();
 
+  // It's OK to initialize these member variables here because the
+  // constructor calls this method.
+
   /** @private {remoting.Host} */
   this.host_ = null;
 
@@ -87,8 +90,6 @@ remoting.SessionConnectorImpl.prototype.resetConnection_ = function() {
   /** @private {remoting.ClientSession} */
   this.clientSession_ = null;
 
-  /** @private {XMLHttpRequest} */
-  this.pendingXhr_ = null;
 
   /** @private {remoting.CredentialsProvider} */
   this.credentialsProvider_ = null;
diff --git a/remoting/webapp/crd/js/third_party_token_fetcher.js b/remoting/webapp/crd/js/third_party_token_fetcher.js
index b51e1b1..66189e9 100644
--- a/remoting/webapp/crd/js/third_party_token_fetcher.js
+++ b/remoting/webapp/crd/js/third_party_token_fetcher.js
@@ -132,7 +132,7 @@ remoting.ThirdPartyTokenFetcher.prototype.parseRedirectUrl_ =
  * @private
  */
 remoting.ThirdPartyTokenFetcher.prototype.getFullTokenUrl_ = function() {
-  return this.tokenUrl_ + '?' + remoting.xhr.urlencodeParamHash({
+  return this.tokenUrl_ + '?' + remoting.Xhr.urlencodeParamHash({
     'redirect_uri': this.redirectUri_,
     'scope': this.tokenScope_,
     'client_id': this.hostPublicKey_,
diff --git a/remoting/webapp/crd/js/xhr.js b/remoting/webapp/crd/js/xhr.js
index 0529838..cdefb0aa 100644
--- a/remoting/webapp/crd/js/xhr.js
+++ b/remoting/webapp/crd/js/xhr.js
@@ -4,7 +4,7 @@
 
 /**
  * @fileoverview
- * Simple utilities for making XHRs more pleasant.
+ * Utility class for making XHRs more pleasant.
  */
 
 'use strict';
@@ -12,26 +12,83 @@
 /** @suppress {duplicate} */
 var remoting = remoting || {};
 
-/** Namespace for XHR functions */
-/** @type {Object} */
-remoting.xhr = remoting.xhr || {};
-
 /**
- * Takes an associative array of parameters and urlencodes it.
- *
- * @param {Object<string,string>} paramHash The parameter key/value pairs.
- * @return {string} URLEncoded version of paramHash.
+ * @constructor
+ * @param {remoting.Xhr.Params} params
  */
-remoting.xhr.urlencodeParamHash = function(paramHash) {
-  var paramArray = [];
-  for (var key in paramHash) {
-    paramArray.push(encodeURIComponent(key) +
-                     '=' + encodeURIComponent(paramHash[key]));
+remoting.Xhr = function(params) {
+  /** @private @const {!XMLHttpRequest} */
+  this.nativeXhr_ = new XMLHttpRequest();
+  this.nativeXhr_.onreadystatechange = this.onReadyStateChange_.bind(this);
+  this.nativeXhr_.withCredentials = params.withCredentials || false;
+
+  /** @private @const */
+  this.responseType_ = params.responseType || remoting.Xhr.ResponseType.TEXT;
+
+  // Apply URL parameters.
+  var url = params.url;
+  var parameterString = '';
+  if (typeof(params.urlParams) === 'string') {
+    parameterString = params.urlParams;
+  } else if (typeof(params.urlParams) === 'object') {
+    parameterString = remoting.Xhr.urlencodeParamHash(
+        remoting.Xhr.removeNullFields_(params.urlParams));
   }
-  if (paramArray.length > 0) {
-    return paramArray.join('&');
+  if (parameterString) {
+    base.debug.assert(url.indexOf('?') == -1);
+    url += '?' + parameterString;
   }
-  return '';
+
+  // Check that the content spec is consistent.
+  if ((Number(params.textContent !== undefined) +
+       Number(params.formContent !== undefined) +
+       Number(params.jsonContent !== undefined)) > 1) {
+    throw new Error(
+        'may only specify one of textContent, formContent, and jsonContent');
+  }
+
+  // Prepare the build modified headers.
+  var headers = remoting.Xhr.removeNullFields_(params.headers);
+
+  // Convert the content fields to a single text content variable.
+  /** @private {?string} */
+  this.content_ = null;
+  if (params.textContent !== undefined) {
+    this.content_ = params.textContent;
+  } else if (params.formContent !== undefined) {
+    if (!('Content-type' in headers)) {
+      headers['Content-type'] = 'application/x-www-form-urlencoded';
+    }
+    this.content_ = remoting.Xhr.urlencodeParamHash(params.formContent);
+  } else if (params.jsonContent !== undefined) {
+    if (!('Content-type' in headers)) {
+      headers['Content-type'] = 'application/json; charset=UTF-8';
+    }
+    this.content_ = JSON.stringify(params.jsonContent);
+  }
+
+  // Apply the oauthToken field.
+  if (params.oauthToken !== undefined) {
+    base.debug.assert(!('Authorization' in headers));
+    headers['Authorization'] = 'Bearer ' + params.oauthToken;
+  }
+
+  this.nativeXhr_.open(params.method, url, true);
+  for (var key in headers) {
+    this.nativeXhr_.setRequestHeader(key, headers[key]);
+  }
+
+  /** @private {base.Deferred<!remoting.Xhr.Response>} */
+  this.deferred_ = null;
+};
+
+/**
+ * @enum {string}
+ */
+remoting.Xhr.ResponseType = {
+  TEXT: 'TEXT',  // Request a plain text response (default).
+  JSON: 'JSON',  // Request a JSON response.
+  NONE: 'NONE'   // Don't request any response.
 };
 
 /**
@@ -41,8 +98,6 @@ remoting.xhr.urlencodeParamHash = function(paramHash) {
  *
  * url: The URL to request.
  *
- * onDone: Function to call when the XHR finishes.
-
  * urlParams: (optional) Parameters to be appended to the URL.
  *     Null-valued parameters are omitted.
  *
@@ -64,20 +119,118 @@ remoting.xhr.urlencodeParamHash = function(paramHash) {
  * oauthToken: (optional) An OAuth2 token used to construct an
  *     Authentication header.
  *
+ * responseType: (optional) Request a response of a specific
+ *    type. Default: TEXT.
+ *
  * @typedef {{
  *   method: string,
  *   url:string,
- *   onDone:(function(XMLHttpRequest):void),
  *   urlParams:(string|Object<string,?string>|undefined),
  *   textContent:(string|undefined),
  *   formContent:(Object|undefined),
  *   jsonContent:(*|undefined),
  *   headers:(Object<string,?string>|undefined),
  *   withCredentials:(boolean|undefined),
- *   oauthToken:(string|undefined)
+ *   oauthToken:(string|undefined),
+ *   responseType:(remoting.Xhr.ResponseType|undefined)
  * }}
  */
-remoting.XhrParams;
+remoting.Xhr.Params;
+
+/**
+ * Aborts the HTTP request.  Does nothing is the request has finished
+ * already.
+ */
+remoting.Xhr.prototype.abort = function() {
+  this.nativeXhr_.abort();
+};
+
+/**
+ * Starts and HTTP request and gets a promise that is resolved when
+ * the request completes.
+ *
+ * Any error that prevents receiving an HTTP status
+ * code causes this promise to be rejected.
+ *
+ * NOTE: Calling this method more than once will return the same
+ * promise and not start a new request, despite what the name
+ * suggests.
+ *
+ * @return {!Promise<!remoting.Xhr.Response>}
+ */
+remoting.Xhr.prototype.start = function() {
+  if (this.deferred_ == null) {
+    var xhr = this.nativeXhr_;
+    xhr.send(this.content_);
+    this.content_ = null;  // for gc
+    this.deferred_ = new base.Deferred();
+  }
+  return this.deferred_.promise();
+};
+
+/**
+ * @private
+ */
+remoting.Xhr.prototype.onReadyStateChange_ = function() {
+  var xhr = this.nativeXhr_;
+  if (xhr.readyState == 4) {
+    // See comments at remoting.Xhr.Response.
+    this.deferred_.resolve(new remoting.Xhr.Response(xhr, this.responseType_));
+  }
+};
+
+/**
+ * The response-related parts of an XMLHttpRequest.  Note that this
+ * class is not just a facade for XMLHttpRequest; it saves the value
+ * of the |responseText| field becuase once onReadyStateChange_
+ * (above) returns, the value of |responseText| is reset to the empty
+ * string!  This is a documented anti-feature of the XMLHttpRequest
+ * API.
+ *
+ * @constructor
+ * @param {!XMLHttpRequest} xhr
+ * @param {remoting.Xhr.ResponseType} type
+ */
+remoting.Xhr.Response = function(xhr, type) {
+  /** @private @const */
+  this.type_ = type;
+
+  /**
+   * The HTTP status code.
+   * @const {number}
+   */
+  this.status = xhr.status;
+
+  /**
+   * The HTTP status description.
+   * @const {string}
+   */
+  this.statusText = xhr.statusText;
+
+  /**
+   * The response URL, if any.
+   * @const {?string}
+   */
+  this.url = xhr.responseURL;
+
+  /** @private {string} */
+  this.text_ = xhr.responseText || '';
+};
+
+/**
+ * @return {string} The text content of the response.
+ */
+remoting.Xhr.Response.prototype.getText = function() {
+  return this.text_;
+};
+
+/**
+ * @return {*} The parsed JSON content of the response.
+ */
+remoting.Xhr.Response.prototype.getJson = function() {
+  base.debug.assert(this.type_ == remoting.Xhr.ResponseType.JSON);
+  return JSON.parse(this.text_);
+};
 
 /**
  * Returns a copy of the input object with all null or undefined
@@ -87,7 +240,7 @@ remoting.XhrParams;
  * @return {!Object<string,string>}
  * @private
  */
-remoting.xhr.removeNullFields_ = function(input) {
+remoting.Xhr.removeNullFields_ = function(input) {
   /** @type {!Object<string,string>} */
   var result = {};
   if (input) {
@@ -102,112 +255,38 @@ remoting.xhr.removeNullFields_ = function(input) {
 };
 
 /**
- * Executes an arbitrary HTTP method asynchronously.
+ * Takes an associative array of parameters and urlencodes it.
  *
- * @param {remoting.XhrParams} params
- * @return {XMLHttpRequest} The XMLHttpRequest object.
+ * @param {Object<string,string>} paramHash The parameter key/value pairs.
+ * @return {string} URLEncoded version of paramHash.
  */
-remoting.xhr.start = function(params) {
-  // Extract fields that can be used more or less as-is.
-  var method = params.method;
-  var url = params.url;
-  var onDone = params.onDone;
-  var headers = remoting.xhr.removeNullFields_(params.headers);
-  var withCredentials = params.withCredentials || false;
-
-  // Apply URL parameters.
-  var parameterString = '';
-  if (typeof(params.urlParams) === 'string') {
-    parameterString = params.urlParams;
-  } else if (typeof(params.urlParams) === 'object') {
-    parameterString = remoting.xhr.urlencodeParamHash(
-        remoting.xhr.removeNullFields_(params.urlParams));
-  }
-  if (parameterString) {
-    base.debug.assert(url.indexOf('?') == -1);
-    url += '?' + parameterString;
-  }
-
-  // Check that the content spec is consistent.
-  if ((Number(params.textContent !== undefined) +
-       Number(params.formContent !== undefined) +
-       Number(params.jsonContent !== undefined)) > 1) {
-    throw new Error(
-        'may only specify one of textContent, formContent, and jsonContent');
-  }
-
-  // Convert the content fields to a single text content variable.
-  /** @type {?string} */
-  var content = null;
-  if (params.textContent !== undefined) {
-    content = params.textContent;
-  } else if (params.formContent !== undefined) {
-    if (!('Content-type' in headers)) {
-      headers['Content-type'] = 'application/x-www-form-urlencoded';
-    }
-    content = remoting.xhr.urlencodeParamHash(params.formContent);
-  } else if (params.jsonContent !== undefined) {
-    if (!('Content-type' in headers)) {
-      headers['Content-type'] = 'application/json; charset=UTF-8';
-    }
-    content = JSON.stringify(params.jsonContent);
-  }
-
-  // Apply the oauthToken field.
-  if (params.oauthToken !== undefined) {
-    base.debug.assert(!('Authorization' in headers));
-    headers['Authorization'] = 'Bearer ' + params.oauthToken;
+remoting.Xhr.urlencodeParamHash = function(paramHash) {
+  var paramArray = [];
+  for (var key in paramHash) {
+    paramArray.push(encodeURIComponent(key) +
+                     '=' + encodeURIComponent(paramHash[key]));
   }
-
-  return remoting.xhr.startInternal_(
-      method, url, onDone, content, headers, withCredentials);
-};
-
-/**
- * Executes an arbitrary HTTP method asynchronously.
- *
- * @param {string} method
- * @param {string} url
- * @param {function(XMLHttpRequest):void} onDone
- * @param {?string} content
- * @param {!Object<string,string>} headers
- * @param {boolean} withCredentials
- * @return {XMLHttpRequest} The XMLHttpRequest object.
- * @private
- */
-remoting.xhr.startInternal_ = function(
-    method, url, onDone, content, headers, withCredentials) {
-  /** @type {XMLHttpRequest} */
-  var xhr = new XMLHttpRequest();
-  xhr.onreadystatechange = function() {
-    if (xhr.readyState != 4) {
-      return;
-    }
-    onDone(xhr);
-  };
-
-  xhr.open(method, url, true);
-  for (var key in headers) {
-    xhr.setRequestHeader(key, headers[key]);
+  if (paramArray.length > 0) {
+    return paramArray.join('&');
   }
-  xhr.withCredentials = withCredentials;
-  xhr.send(content);
-  return xhr;
+  return '';
 };
 
 /**
  * Generic success/failure response proxy.
  *
+ * TODO(jrw): Stop using this and move default error handling directly
+ * into Xhr class.
+ *
  * @param {function():void} onDone
  * @param {function(!remoting.Error):void} onError
  * @param {Array<remoting.Error.Tag>=} opt_ignoreErrors
- * @return {function(XMLHttpRequest):void}
+ * @return {function(!remoting.Xhr.Response):void}
  */
-remoting.xhr.defaultResponse = function(onDone, onError, opt_ignoreErrors) {
-  /** @param {XMLHttpRequest} xhr */
-  var result = function(xhr) {
-    var error =
-        remoting.Error.fromHttpStatus(/** @type {number} */ (xhr.status));
+remoting.Xhr.defaultResponse = function(onDone, onError, opt_ignoreErrors) {
+  /** @param {!remoting.Xhr.Response} response */
+  var result = function(response) {
+    var error = remoting.Error.fromHttpStatus(response.status);
     if (error.isNone()) {
       onDone();
       return;
diff --git a/remoting/webapp/crd/js/xhr_unittest.js b/remoting/webapp/crd/js/xhr_unittest.js
index 7102c2e..b02987e 100644
--- a/remoting/webapp/crd/js/xhr_unittest.js
+++ b/remoting/webapp/crd/js/xhr_unittest.js
@@ -4,193 +4,185 @@
 
 /**
  * @fileoverview
- * @suppress {checkTypes|checkVars|reportUnknownTypes}
  */
 
 (function() {
 
 'use strict';
-QUnit.module('xhr');
+
+/** @type {sinon.FakeXhr} */
+var fakeXhr;
+
+QUnit.module('xhr', {
+  beforeEach: function() {
+    fakeXhr = null;
+    sinon.useFakeXMLHttpRequest().onCreate =
+        function(/** sinon.FakeXhr */ xhr) {
+          fakeXhr = xhr;
+        };
+  }
+});
 
 QUnit.test('urlencodeParamHash', function(assert) {
   assert.equal(
-      remoting.xhr.urlencodeParamHash({}),
+      remoting.Xhr.urlencodeParamHash({}),
       '');
   assert.equal(
-      remoting.xhr.urlencodeParamHash({'key': 'value'}),
+      remoting.Xhr.urlencodeParamHash({'key': 'value'}),
       'key=value');
   assert.equal(
-      remoting.xhr.urlencodeParamHash({'key /?=&': 'value /?=&'}),
+      remoting.Xhr.urlencodeParamHash({'key /?=&': 'value /?=&'}),
       'key%20%2F%3F%3D%26=value%20%2F%3F%3D%26');
   assert.equal(
-      remoting.xhr.urlencodeParamHash({'k1': 'v1', 'k2': 'v2'}),
+      remoting.Xhr.urlencodeParamHash({'k1': 'v1', 'k2': 'v2'}),
       'k1=v1&k2=v2');
 });
 
 QUnit.test('basic GET', function(assert) {
-  sinon.useFakeXMLHttpRequest();
-  var done = assert.async();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'GET',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    }
+    responseType: remoting.Xhr.ResponseType.TEXT
+  }).start().then(function(response) {
+      assert.equal(response.status, 200);
+      assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'GET');
-  assert.equal(request.url, 'http://foo.com');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, null);
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.equal(fakeXhr.method, 'GET');
+  assert.equal(fakeXhr.url, 'http://foo.com');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, null);
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('GET with param string', function(assert) {
-  var done = assert.async();
-  sinon.useFakeXMLHttpRequest();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'GET',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     urlParams: 'the_param_string'
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'GET');
-  assert.equal(request.url, 'http://foo.com?the_param_string');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, null);
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.equal(fakeXhr.method, 'GET');
+  assert.equal(fakeXhr.url, 'http://foo.com?the_param_string');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, null);
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('GET with param object', function(assert) {
-  var done = assert.async();
-  sinon.useFakeXMLHttpRequest();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'GET',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     urlParams: {'a': 'b', 'c': 'd'}
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'GET');
-  assert.equal(request.url, 'http://foo.com?a=b&c=d');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, null);
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.equal(fakeXhr.method, 'GET');
+  assert.equal(fakeXhr.url, 'http://foo.com?a=b&c=d');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, null);
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('GET with headers', function(assert) {
-  sinon.useFakeXMLHttpRequest();
-  var done = assert.async();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'GET',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     headers: {'Header1': 'headerValue1', 'Header2': 'headerValue2'}
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'GET');
-  assert.equal(request.url, 'http://foo.com');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, null);
+  assert.equal(fakeXhr.method, 'GET');
+  assert.equal(fakeXhr.url, 'http://foo.com');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, null);
   assert.equal(
-      request.requestHeaders['Header1'],
+      fakeXhr.requestHeaders['Header1'],
       'headerValue1');
   assert.equal(
-      request.requestHeaders['Header2'],
+      fakeXhr.requestHeaders['Header2'],
       'headerValue2');
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 
 QUnit.test('GET with credentials', function(assert) {
-  sinon.useFakeXMLHttpRequest();
-  var done = assert.async();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'GET',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     withCredentials: true
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'GET');
-  assert.equal(request.url, 'http://foo.com');
-  assert.equal(request.withCredentials, true);
-  assert.equal(request.requestBody, null);
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.equal(fakeXhr.method, 'GET');
+  assert.equal(fakeXhr.url, 'http://foo.com');
+  assert.equal(fakeXhr.withCredentials, true);
+  assert.equal(fakeXhr.requestBody, null);
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('POST with text content', function(assert) {
-  sinon.useFakeXMLHttpRequest();
   var done = assert.async();
-  var request = remoting.xhr.start({
+
+  var promise = new remoting.Xhr({
     method: 'POST',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     textContent: 'the_content_string'
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
+    done();
   });
-  assert.equal(request.method, 'POST');
-  assert.equal(request.url, 'http://foo.com');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, 'the_content_string');
-  assert.ok(!('Content-type' in request.requestHeaders));
-  request.respond(200, {}, 'body');
+  assert.equal(fakeXhr.method, 'POST');
+  assert.equal(fakeXhr.url, 'http://foo.com');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, 'the_content_string');
+  assert.ok(!('Content-type' in fakeXhr.requestHeaders));
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('POST with form content', function(assert) {
-  sinon.useFakeXMLHttpRequest();
-  var done = assert.async();
-  var request = remoting.xhr.start({
+  var promise = new remoting.Xhr({
     method: 'POST',
     url: 'http://foo.com',
-    onDone: function(xhr) {
-      assert.ok(xhr === request);
-      assert.equal(xhr.status, 200);
-      assert.equal(xhr.responseText, 'body');
-      done();
-    },
+    responseType: remoting.Xhr.ResponseType.TEXT,
     formContent: {'a': 'b', 'c': 'd'}
+  }).start().then(function(response) {
+    assert.equal(response.status, 200);
+    assert.equal(response.getText(), 'body');
   });
-  assert.equal(request.method, 'POST');
-  assert.equal(request.url, 'http://foo.com');
-  assert.equal(request.withCredentials, false);
-  assert.equal(request.requestBody, 'a=b&c=d');
+  assert.equal(fakeXhr.method, 'POST');
+  assert.equal(fakeXhr.url, 'http://foo.com');
+  assert.equal(fakeXhr.withCredentials, false);
+  assert.equal(fakeXhr.requestBody, 'a=b&c=d');
   assert.equal(
-      request.requestHeaders['Content-type'],
+      fakeXhr.requestHeaders['Content-type'],
       'application/x-www-form-urlencoded');
-  request.respond(200, {}, 'body');
+  fakeXhr.respond(200, {}, 'body');
+  return promise;
 });
 
 QUnit.test('defaultResponse 200', function(assert) {
-  sinon.useFakeXMLHttpRequest();
   var done = assert.async();
 
   var onDone = function() {
@@ -203,18 +195,17 @@ QUnit.test('defaultResponse 200', function(assert) {
     done();
   };
 
-  var request = remoting.xhr.start({
+  new remoting.Xhr({
     method: 'POST',
-    url: 'http://foo.com',
-    onDone: remoting.xhr.defaultResponse(onDone, onError)
-  });
-  request.respond(200);
+    url: 'http://foo.com'
+  }).start().then(remoting.Xhr.defaultResponse(onDone, onError));
+  fakeXhr.respond(200, {}, '');
 });
 
 
 QUnit.test('defaultResponse 404', function(assert) {
-  sinon.useFakeXMLHttpRequest();
   var done = assert.async();
+
   var onDone = function() {
     assert.ok(false);
     done();
@@ -225,12 +216,11 @@ QUnit.test('defaultResponse 404', function(assert) {
     done();
   };
 
-  var request = remoting.xhr.start({
+  new remoting.Xhr({
     method: 'POST',
-    url: 'http://foo.com',
-    onDone: remoting.xhr.defaultResponse(onDone, onError)
-  });
-  request.respond(404);
+    url: 'http://foo.com'
+  }).start().then(remoting.Xhr.defaultResponse(onDone, onError));
+  fakeXhr.respond(404, {}, '');
 });
 
 })();
diff --git a/remoting/webapp/js_proto/qunit_proto.js b/remoting/webapp/js_proto/qunit_proto.js
index aed670e..33b30f7 100644
--- a/remoting/webapp/js_proto/qunit_proto.js
+++ b/remoting/webapp/js_proto/qunit_proto.js
@@ -100,4 +100,4 @@ QUnit.module = function(desc, opt_args) {};
  * @param {string} desc
  * @param {function(QUnit.Assert)} f
  */
-QUnit.test = function(desc, f) {};
\ No newline at end of file
+QUnit.test = function(desc, f) {};
diff --git a/remoting/webapp/js_proto/sinon_proto.js b/remoting/webapp/js_proto/sinon_proto.js
index ff7eb96..9e223dc 100644
--- a/remoting/webapp/js_proto/sinon_proto.js
+++ b/remoting/webapp/js_proto/sinon_proto.js
@@ -126,3 +126,36 @@ sinon.TestStub.prototype.onFirstCall = function() {};
 
 /** @returns {Object}  */
 sinon.createStubInstance = function (/** * */ constructor) {};
+
+/** @return {sinon.FakeXhr} */
+sinon.useFakeXMLHttpRequest = function() {};
+
+/** @interface */
+sinon.FakeXhr = function() {};
+
+/** @type {string} */
+sinon.FakeXhr.prototype.method;
+
+/** @type {string} */
+sinon.FakeXhr.prototype.url;
+
+/** @type {boolean} */
+sinon.FakeXhr.prototype.withCredentials;
+
+/** @type {?string} */
+sinon.FakeXhr.prototype.requestBody;
+
+/** @type {!Object<string,string>} */
+sinon.FakeXhr.prototype.requestHeaders;
+
+/**
+ * @param {number} status
+ * @param {!Object<string,string>} headers
+ * @param {?string} content
+ */
+sinon.FakeXhr.prototype.respond;
+
+/**
+ * @type {?function(!sinon.FakeXhr)}
+ */
+sinon.FakeXhr.prototype.onCreate;
\ No newline at end of file