This replaces AutomationTree with AutomationRootNode, which is a regular node which happens to be the root of the tree as well, and contains the lookup table for all other nodes etc.

BUG=309681
TBR=jam@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/311913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276096 0039d316-1c4b-4281-b951-d872f2087c98

21 files changed, 450 insertions, 301 deletions

diff --git a/chrome/chrome_tests_unit.gypi b/chrome/chrome_tests_unit.gypi
index 7ac3fa1..88188d7 100644
--- a/chrome/chrome_tests_unit.gypi
+++ b/chrome/chrome_tests_unit.gypi
@@ -1916,6 +1916,7 @@
         'renderer/extensions/module_system_unittest.cc',
         'renderer/extensions/renderer_permissions_policy_delegate_unittest.cc',
         'renderer/extensions/safe_builtins_unittest.cc',
+        'renderer/extensions/utils_unittest.cc',
         'renderer/media/cast_ipc_dispatcher_unittest.cc',
         'renderer/media/chrome_webrtc_log_message_delegate_unittest.cc',
         'renderer/net/net_error_helper_core_unittest.cc',
diff --git a/chrome/common/extensions/api/automation.idl b/chrome/common/extensions/api/automation.idl
index 11de2d1..6c9e8f8 100644
--- a/chrome/common/extensions/api/automation.idl
+++ b/chrome/common/extensions/api/automation.idl
@@ -31,8 +31,14 @@
   // A listener for events on an <code>AutomationNode</code>.
   callback AutomationListener = void(AutomationEvent event);
 
-  // A single node in an <code>AutomationTree</code>.
+  // A single node in an Automation tree.
   [nocompile] dictionary AutomationNode {
+    // The root node of the tree containing this AutomationNode.
+    AutomationRootNode root;
+
+    // Whether this AutomationNode is an AutomationRootNode.
+    bool isRootNode;
+
     // Unique ID to identify this node.
     long id;
 
@@ -75,18 +81,34 @@
         DOMString eventType, AutomationListener listener, bool capture);
   };
 
-  // The automation tree for a single page.
-  [nocompile] dictionary AutomationTree {
-    AutomationNode root;
-
-    static void addEventListener(
-        DOMString eventType, AutomationListener listener, bool capture);
-    static void removeEventListener(
-        DOMString eventType, AutomationListener listener, bool capture);
+  // The root node of the automation tree for a single frame or desktop.
+  // This may be:
+  // <ul>
+  //   <li> The desktop
+  //   <li> The top frame of a page
+  //   <li> A frame or iframe within a page
+  // </ul>
+  // Thus, an <code>AutomationRootNode</code> may be a descendant of one or
+  // more <code>AutomationRootNode</code>s, and in turn have one or more
+  // <code>AutomationRootNode</code>s in its descendants. Thus, the
+  // <code>root</code> property of the <code>AutomationRootNode</code> will be
+  // the immediate parent <code>AutomationRootNode</code>, or <code>null</code>
+  // if this is the top-level <code>AutomationRootNode</code>.
+  //
+  // Extends $(ref:automation.AutomationNode).
+  [nocompile] dictionary AutomationRootNode {
+    // Whether this AutomationRootNode is loaded or not. If false, call load()
+    // to get the contents.
+    // TODO(aboxhall/dtseng): implement.
+    bool loaded;
+
+    // Load the accessibility tree for this AutomationRootNode.
+    // TODO(aboxhall/dtseng): implement.
+    static void load(RootCallback callback);
   };
 
-  // Called when the <code>AutomationTree</code> for the page is available.
-  callback RootCallback = void(AutomationTree tree);
+  // Called when the <code>AutomationRootNode</code> for the page is available.
+  callback RootCallback = void(AutomationRootNode rootNode);
 
   interface Functions {
     // Get the automation tree for the tab with the given tabId, or the current
diff --git a/chrome/renderer/extensions/chrome_extensions_dispatcher_delegate.cc b/chrome/renderer/extensions/chrome_extensions_dispatcher_delegate.cc
index c8e411e..04db8f1 100644
--- a/chrome/renderer/extensions/chrome_extensions_dispatcher_delegate.cc
+++ b/chrome/renderer/extensions/chrome_extensions_dispatcher_delegate.cc
@@ -160,7 +160,6 @@ void ChromeExtensionsDispatcherDelegate::PopulateSourceMap(
   source_map->RegisterSource("automation", IDR_AUTOMATION_CUSTOM_BINDINGS_JS);
   source_map->RegisterSource("automationEvent", IDR_AUTOMATION_EVENT_JS);
   source_map->RegisterSource("automationNode", IDR_AUTOMATION_NODE_JS);
-  source_map->RegisterSource("automationTree", IDR_AUTOMATION_TREE_JS);
   source_map->RegisterSource("browserAction",
                              IDR_BROWSER_ACTION_CUSTOM_BINDINGS_JS);
   source_map->RegisterSource("declarativeContent",
diff --git a/chrome/renderer/extensions/utils_unittest.cc b/chrome/renderer/extensions/utils_unittest.cc
new file mode 100644
index 0000000..3a718dd
--- /dev/null
+++ b/chrome/renderer/extensions/utils_unittest.cc
@@ -0,0 +1,123 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/strings/stringprintf.h"
+#include "chrome/test/base/module_system_test.h"
+#include "grit/extensions_renderer_resources.h"
+
+namespace extensions {
+namespace {
+
+class UtilsUnittest : public ModuleSystemTest {
+ protected:
+  void RegisterTestModule(const char* code) {
+    RegisterModule("test",
+                   base::StringPrintf(
+                       "var assert = requireNative('assert');\n"
+                       "var AssertTrue = assert.AssertTrue;\n"
+                       "var AssertFalse = assert.AssertFalse;\n"
+                       "var utils = require('utils');\n"
+                       "%s",
+                       code));
+  }
+
+ private:
+  virtual void SetUp() OVERRIDE {
+    ModuleSystemTest::SetUp();
+
+    RegisterModule("utils", IDR_UTILS_JS);
+    OverrideNativeHandler("schema_registry",
+                          "exports.GetSchema = function() {};");
+    OverrideNativeHandler("logging",
+                          "exports.CHECK = function() {};\n"
+                          "exports.WARNING = function() {};");
+  }
+};
+
+TEST_F(UtilsUnittest, TestNothing) {
+  ExpectNoAssertionsMade();
+}
+
+TEST_F(UtilsUnittest, SuperClass) {
+  ModuleSystem::NativesEnabledScope natives_enabled_scope(
+      context_->module_system());
+  RegisterTestModule(
+      "function SuperClassImpl() {}\n"
+      "\n"
+      "SuperClassImpl.prototype = {\n"
+      "  attrA: 'aSuper',\n"
+      "  attrB: 'bSuper',\n"
+      "  func: function() { return 'func'; },\n"
+      "  superFunc: function() { return 'superFunc'; }\n"
+      "};\n"
+      "\n"
+      "function SubClassImpl() {\n"
+      "  SuperClassImpl.call(this);\n"
+      "}\n"
+      "\n"
+      "SubClassImpl.prototype = {\n"
+      "  __proto__: SuperClassImpl.prototype,\n"
+      "  attrA: 'aSub',\n"
+      "  attrC: 'cSub',\n"
+      "  func: function() { return 'overridden'; },\n"
+      "  subFunc: function() { return 'subFunc'; }\n"
+      "};\n"
+      "\n"
+      "var SuperClass = utils.expose('SuperClass',\n"
+      "                              SuperClassImpl,\n"
+      "                              { functions: ['func', 'superFunc'],\n"
+      "                                properties: ['attrA', 'attrB'] });\n"
+      "\n"
+      "var SubClass = utils.expose('SubClass',\n"
+      "                            SubClassImpl,\n"
+      "                            { superclass: SuperClass,\n"
+      "                              functions: ['subFunc'],\n"
+      "                              properties: ['attrC'] });\n"
+      "\n"
+      "var supe = new SuperClass();\n"
+      "AssertTrue(supe.attrA == 'aSuper');\n"
+      "AssertTrue(supe.attrB == 'bSuper');\n"
+      "AssertFalse('attrC' in supe);\n"
+      "AssertTrue(supe.func() == 'func');\n"
+      "AssertTrue('superFunc' in supe);\n"
+      "AssertTrue(supe.superFunc() == 'superFunc');\n"
+      "AssertFalse('subFunc' in supe);\n"
+      "AssertTrue(supe instanceof SuperClass);\n"
+      "\n"
+      "var sub = new SubClass();\n"
+      "AssertTrue(sub.attrA == 'aSub');\n"
+      "AssertTrue(sub.attrB == 'bSuper');\n"
+      "AssertTrue(sub.attrC == 'cSub');\n"
+      "AssertTrue(sub.func() == 'overridden');\n"
+      "AssertTrue(sub.superFunc() == 'superFunc');\n"
+      "AssertTrue('subFunc' in sub);\n"
+      "AssertTrue(sub.subFunc() == 'subFunc');\n"
+      "AssertTrue(sub instanceof SuperClass);\n"
+      "AssertTrue(sub instanceof SubClass);\n"
+      "\n"
+      "function SubSubClassImpl() {}\n"
+      "SubSubClassImpl.prototype = Object.create(SubClassImpl.prototype);\n"
+      "SubSubClassImpl.prototype.subSubFunc = function() { return 'subsub'; }\n"
+      "\n"
+      "var SubSubClass = utils.expose('SubSubClass',\n"
+      "                               SubSubClassImpl,\n"
+      "                               { superclass: SubClass,\n"
+      "                                 functions: ['subSubFunc'] });\n"
+      "var subsub = new SubSubClass();\n"
+      "AssertTrue(subsub.attrA == 'aSub');\n"
+      "AssertTrue(subsub.attrB == 'bSuper');\n"
+      "AssertTrue(subsub.attrC == 'cSub');\n"
+      "AssertTrue(subsub.func() == 'overridden');\n"
+      "AssertTrue(subsub.superFunc() == 'superFunc');\n"
+      "AssertTrue(subsub.subFunc() == 'subFunc');\n"
+      "AssertTrue(subsub.subSubFunc() == 'subsub');\n"
+      "AssertTrue(subsub instanceof SuperClass);\n"
+      "AssertTrue(subsub instanceof SubClass);\n"
+      "AssertTrue(subsub instanceof SubSubClass);\n");
+
+  context_->module_system()->Require("test");
+}
+
+}  // namespace
+}  // namespace extensions
diff --git a/chrome/renderer/resources/extensions/automation/automation_node.js b/chrome/renderer/resources/extensions/automation/automation_node.js
index 944afc8..838de6e 100644
--- a/chrome/renderer/resources/extensions/automation/automation_node.js
+++ b/chrome/renderer/resources/extensions/automation/automation_node.js
@@ -11,42 +11,46 @@ var IsInteractPermitted =
 
 /**
  * A single node in the Automation tree.
- * @param {AutomationTree} owner The owning tree.
+ * @param {AutomationRootNodeImpl} root The root of the tree.
  * @constructor
  */
-var AutomationNodeImpl = function(owner) {
-  this.owner = owner;
+function AutomationNodeImpl(root) {
+  this.rootImpl = root;
   this.childIds = [];
   this.attributes = {};
   this.listeners = {};
   this.location = { left: 0, top: 0, width: 0, height: 0 };
-};
+}
 
 AutomationNodeImpl.prototype = {
   id: -1,
   role: '',
-  loaded: false,
   state: { busy: true },
+  isRootNode: false,
+
+  get root() {
+    return this.rootImpl.wrapper;
+  },
 
   parent: function() {
-    return this.owner.get(this.parentID);
+    return this.rootImpl.get(this.parentID);
   },
 
   firstChild: function() {
-    var node = this.owner.get(this.childIds[0]);
+    var node = this.rootImpl.get(this.childIds[0]);
     return node;
   },
 
   lastChild: function() {
     var childIds = this.childIds;
-    var node = this.owner.get(childIds[childIds.length - 1]);
+    var node = this.rootImpl.get(childIds[childIds.length - 1]);
     return node;
   },
 
   children: function() {
     var children = [];
     for (var i = 0, childID; childID = this.childIds[i]; i++)
-      children.push(this.owner.get(childID));
+      children.push(this.rootImpl.get(childID));
     return children;
   },
 
@@ -86,7 +90,7 @@ AutomationNodeImpl.prototype = {
     this.removeEventListener(eventType, callback);
     if (!this.listeners[eventType])
       this.listeners[eventType] = [];
-    this.listeners[eventType].push({callback: callback, capture: capture});
+    this.listeners[eventType].push({callback: callback, capture: !!capture});
   },
 
   // TODO(dtseng/aboxhall): Check this impl against spec.
@@ -108,7 +112,6 @@ AutomationNodeImpl.prototype = {
       // TODO(aboxhall/dtseng): handle unloaded parent node
       parent = parent.parent();
     }
-    path.push(this.owner.wrapper);
     var event = new AutomationEvent(eventType, this.wrapper);
 
     // Dispatch the event through the propagation path in three phases:
@@ -162,7 +165,6 @@ AutomationNodeImpl.prototype = {
     var listeners = nodeImpl.listeners[event.type];
     if (!listeners)
       return;
-
     var eventPhase = event.eventPhase;
     for (var i = 0; i < listeners.length; i++) {
       if (eventPhase == Event.CAPTURING_PHASE && !listeners[i].capture)
@@ -182,8 +184,8 @@ AutomationNodeImpl.prototype = {
 
   performAction_: function(actionType, opt_args) {
     // Not yet initialized.
-    if (this.owner.processID === undefined ||
-        this.owner.routingID === undefined ||
+    if (this.rootImpl.processID === undefined ||
+        this.rootImpl.routingID === undefined ||
         this.wrapper.id === undefined) {
       return;
     }
@@ -194,14 +196,179 @@ AutomationNodeImpl.prototype = {
           ' {"interact": true} in the "automation" manifest key.');
     }
 
-    automationInternal.performAction({ processID: this.owner.processID,
-                                       routingID: this.owner.routingID,
+    automationInternal.performAction({ processID: this.rootImpl.processID,
+                                       routingID: this.rootImpl.routingID,
                                        automationNodeID: this.wrapper.id,
                                        actionType: actionType },
                                      opt_args || {});
   }
 };
 
+// Maps an attribute to its default value in an invalidated node.
+// These attributes are taken directly from the Automation idl.
+var AutomationAttributeDefaults = {
+  'id': -1,
+  'role': '',
+  'state': {},
+  'location': { left: 0, top: 0, width: 0, height: 0 }
+};
+
+
+var AutomationAttributeTypes = [
+  'boolAttributes',
+  'floatAttributes',
+  'htmlAttributes',
+  'intAttributes',
+  'intlistAttributes',
+  'stringAttributes'
+];
+
+
+/**
+ * AutomationRootNode.
+ *
+ * An AutomationRootNode is the javascript end of an AXTree living in the
+ * browser. AutomationRootNode handles unserializing incremental updates from
+ * the source AXTree. Each update contains node data that form a complete tree
+ * after applying the update.
+ *
+ * A brief note about ids used through this class. The source AXTree assigns
+ * unique ids per node and we use these ids to build a hash to the actual
+ * AutomationNode object.
+ * Thus, tree traversals amount to a lookup in our hash.
+ *
+ * The tree itself is identified by the process id and routing id of the
+ * renderer widget host.
+ * @constructor
+ */
+function AutomationRootNodeImpl(processID, routingID) {
+  AutomationNodeImpl.call(this, this);
+  this.processID = processID;
+  this.routingID = routingID;
+  this.axNodeDataCache_ = {};
+}
+
+AutomationRootNodeImpl.prototype = {
+  __proto__: AutomationNodeImpl.prototype,
+
+  isRootNode: true,
+
+  get: function(id) {
+    return this.axNodeDataCache_[id];
+  },
+
+  invalidate: function(node) {
+    if (!node)
+      return;
+
+    var children = node.children();
+
+    for (var i = 0, child; child = children[i]; i++)
+      this.invalidate(child);
+
+    // Retrieve the internal AutomationNodeImpl instance for this node.
+    // This object is not accessible outside of bindings code, but we can access
+    // it here.
+    var nodeImpl = privates(node).impl;
+    var id = node.id;
+    for (var key in AutomationAttributeDefaults) {
+      nodeImpl[key] = AutomationAttributeDefaults[key];
+    }
+    nodeImpl.loaded = false;
+    nodeImpl.id = id;
+    this.axNodeDataCache_[id] = undefined;
+  },
+
+  update: function(data) {
+    var didUpdateRoot = false;
+
+    if (data.nodes.length == 0)
+      return false;
+
+    for (var i = 0; i < data.nodes.length; i++) {
+      var nodeData = data.nodes[i];
+      var node = this.axNodeDataCache_[nodeData.id];
+      if (!node) {
+        if (nodeData.role == 'rootWebArea' || nodeData.role == 'desktop') {
+          // |this| is an AutomationRootNodeImpl; retrieve the
+          // AutomationRootNode instance instead.
+          node = this.wrapper;
+          didUpdateRoot = true;
+        } else {
+          node = new AutomationNode(this);
+        }
+      }
+      var nodeImpl = privates(node).impl;
+
+      // Update children.
+      var oldChildIDs = nodeImpl.childIds;
+      var newChildIDs = nodeData.childIds || [];
+      var newChildIDsHash = {};
+
+      for (var j = 0, newId; newId = newChildIDs[j]; j++) {
+        // Hash the new child ids for faster lookup.
+        newChildIDsHash[newId] = newId;
+
+        // We need to update all new children's parent id regardless.
+        var childNode = this.get(newId);
+        if (!childNode) {
+          childNode = new AutomationNode(this);
+          this.axNodeDataCache_[newId] = childNode;
+          privates(childNode).impl.id = newId;
+        }
+        privates(childNode).impl.indexInParent = j;
+        privates(childNode).impl.parentID = nodeData.id;
+      }
+
+      for (var k = 0, oldId; oldId = oldChildIDs[k]; k++) {
+        // However, we must invalidate all old child ids that are no longer
+        // children.
+        if (!newChildIDsHash[oldId]) {
+          this.invalidate(this.get(oldId));
+        }
+      }
+
+      for (var key in AutomationAttributeDefaults) {
+        if (key in nodeData)
+          nodeImpl[key] = nodeData[key];
+        else
+          nodeImpl[key] = AutomationAttributeDefaults[key];
+      }
+      for (var attributeTypeIndex = 0;
+           attributeTypeIndex < AutomationAttributeTypes.length;
+           attributeTypeIndex++) {
+        var attributeType = AutomationAttributeTypes[attributeTypeIndex];
+        for (var attributeID in nodeData[attributeType]) {
+          nodeImpl.attributes[attributeID] =
+              nodeData[attributeType][attributeID];
+        }
+      }
+      nodeImpl.childIds = newChildIDs;
+      nodeImpl.loaded = true;
+      this.axNodeDataCache_[node.id] = node;
+    }
+    var node = this.get(data.targetID);
+    if (node)
+      nodeImpl.dispatchEvent(data.eventType);
+    return true;
+  },
+
+  toString: function() {
+    function toStringInternal(node, indent) {
+      if (!node)
+        return '';
+      var output =
+        new Array(indent).join(' ') + privates(node).impl.toString() + '\n';
+      indent += 2;
+      for (var i = 0; i < node.children().length; i++)
+        output += toStringInternal(node.children()[i], indent);
+      return output;
+    }
+    return toStringInternal(this, 0);
+  }
+};
+
+
 var AutomationNode = utils.expose('AutomationNode',
                                   AutomationNodeImpl,
                                   { functions: ['parent',
@@ -216,11 +383,18 @@ var AutomationNode = utils.expose('AutomationNode',
                                                 'setSelection',
                                                 'addEventListener',
                                                 'removeEventListener'],
-                                    readonly: ['id',
+                                    readonly: ['isRootNode',
+                                               'id',
                                                'role',
                                                'state',
                                                'location',
-                                               'attributes',
-                                               'loaded'] });
+                                               'attributes'] });
+
+var AutomationRootNode = utils.expose('AutomationRootNode',
+                                      AutomationRootNodeImpl,
+                                      { superclass: AutomationNode,
+                                        functions: ['load'],
+                                        readonly: ['loaded'] });
 
 exports.AutomationNode = AutomationNode;
+exports.AutomationRootNode = AutomationRootNode;
diff --git a/chrome/renderer/resources/extensions/automation/automation_tree.js b/chrome/renderer/resources/extensions/automation/automation_tree.js
deleted file mode 100644
index e7afd4b..0000000
--- a/chrome/renderer/resources/extensions/automation/automation_tree.js
+++ /dev/null
@@ -1,187 +0,0 @@
-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-var Event = require('event_bindings').Event;
-var AutomationNode = require('automationNode').AutomationNode;
-var utils = require('utils');
-
-// Maps an attribute to its default value in an invalidated node.
-// These attributes are taken directly from the Automation idl.
-var AutomationAttributeDefaults = {
-  'id': -1,
-  'role': '',
-  'state': {},
-  'location': { left: 0, top: 0, width: 0, height: 0 }
-};
-
-
-var AutomationAttributeTypes = [
-  'boolAttributes',
-  'floatAttributes',
-  'htmlAttributes',
-  'intAttributes',
-  'intlistAttributes',
-  'stringAttributes'
-];
-
-
-/**
- * AutomationTree.
- *
- * An AutomationTree is the javascript end of an AXTree living in the browser.
- * AutomationTree handles unserializing incremental updates from the source
- * AXTree. Each update contains node data that form a complete tree after
- * applying the update.
- *
- * A brief note about ids used through this class. The source AXTree assigns
- * unique ids per node and we use these ids to build a hash to the actual
- * AutomationNode object.
- * Thus, tree traversals amount to a lookup in our hash.
- *
- * The tree itself is identified by the process id and routing id of the
- * renderer widget host.
- * @constructor
- */
-var AutomationTreeImpl = function(processID, routingID) {
-  this.processID = processID;
-  this.routingID = routingID;
-  this.listeners = {};
-  this.root = new AutomationNode(this);
-  this.axNodeDataCache_ = {};
-};
-
-AutomationTreeImpl.prototype = {
-  root: null,
-
-  get: function(id) {
-    return this.axNodeDataCache_[id];
-  },
-
-  invalidate: function(node) {
-    if (!node)
-      return;
-
-    var children = node.children();
-
-    for (var i = 0, child; child = children[i]; i++)
-      this.invalidate(child);
-
-    var id = node.id;
-    for (var key in AutomationAttributeDefaults) {
-      privates(node).impl[key] = AutomationAttributeDefaults[key];
-    }
-    privates(node).impl.loaded = false;
-    privates(node).impl.id = id;
-    this.axNodeDataCache_[id] = undefined;
-  },
-
-  // TODO(aboxhall): make AutomationTree inherit from AutomationNode (or a
-  // mutual ancestor) and remove this code.
-  addEventListener: function(eventType, callback, capture) {
-    this.removeEventListener(eventType, callback);
-    if (!this.listeners[eventType])
-      this.listeners[eventType] = [];
-    this.listeners[eventType].push({callback: callback, capture: capture});
-  },
-
-  removeEventListener: function(eventType, callback) {
-    if (this.listeners[eventType]) {
-      var listeners = this.listeners[eventType];
-      for (var i = 0; i < listeners.length; i++) {
-        if (callback === listeners[i].callback)
-          listeners.splice(i, 1);
-      }
-    }
-  },
-
-  update: function(data) {
-    var didUpdateRoot = false;
-
-    if (data.nodes.length == 0)
-      return false;
-
-    for (var i = 0; i < data.nodes.length; i++) {
-      var nodeData = data.nodes[i];
-      var node = this.axNodeDataCache_[nodeData.id];
-      if (!node) {
-        node = new AutomationNode(this);
-      }
-
-      // Update children.
-      var oldChildIDs = privates(node).impl.childIds;
-      var newChildIDs = nodeData.childIds || [];
-      var newChildIDsHash = {};
-
-      for (var j = 0, newId; newId = newChildIDs[j]; j++) {
-        // Hash the new child ids for faster lookup.
-        newChildIDsHash[newId] = newId;
-
-        // We need to update all new children's parent id regardless.
-        var childNode = this.get(newId);
-        if (!childNode) {
-          childNode = new AutomationNode(this);
-          this.axNodeDataCache_[newId] = childNode;
-          privates(childNode).impl.id = newId;
-        }
-        privates(childNode).impl.indexInParent = j;
-        privates(childNode).impl.parentID = nodeData.id;
-      }
-
-      for (var k = 0, oldId; oldId = oldChildIDs[k]; k++) {
-        // However, we must invalidate all old child ids that are no longer
-        // children.
-        if (!newChildIDsHash[oldId]) {
-          this.invalidate(this.get(oldId));
-        }
-      }
-
-      if (nodeData.role == 'rootWebArea' || nodeData.role == 'desktop') {
-        this.root = node;
-        didUpdateRoot = true;
-      }
-      for (var key in AutomationAttributeDefaults) {
-        if (key in nodeData)
-          privates(node).impl[key] = nodeData[key];
-        else
-          privates(node).impl[key] = AutomationAttributeDefaults[key];
-      }
-      for (var attributeTypeIndex = 0;
-           attributeTypeIndex < AutomationAttributeTypes.length;
-           attributeTypeIndex++) {
-        var attributeType = AutomationAttributeTypes[attributeTypeIndex];
-        for (var attributeID in nodeData[attributeType]) {
-          privates(node).impl.attributes[attributeID] =
-              nodeData[attributeType][attributeID];
-        }
-      }
-      privates(node).impl.childIds = newChildIDs;
-      privates(node).impl.loaded = true;
-      this.axNodeDataCache_[node.id] = node;
-    }
-    var node = this.get(data.targetID);
-    if (node)
-      privates(node).impl.dispatchEvent(data.eventType);
-    return true;
-  },
-
-  toString: function() {
-    function toStringInternal(node, indent) {
-      if (!node)
-        return '';
-      var output =
-          new Array(indent).join(' ') + privates(node).impl.toString() + '\n';
-      indent += 2;
-      for (var i = 0; i < node.children().length; i++)
-        output += toStringInternal(node.children()[i], indent);
-      return output;
-    }
-    return toStringInternal(this.root, 0);
-  }
-};
-
-exports.AutomationTree = utils.expose('AutomationTree',
-                                      AutomationTreeImpl,
-                                      { functions: ['addEventListener',
-                                                    'removeEventListener'],
-                                        readonly: ['root'] });
diff --git a/chrome/renderer/resources/extensions/automation_custom_bindings.js b/chrome/renderer/resources/extensions/automation_custom_bindings.js
index 96e9e8d..8a0fe48 100644
--- a/chrome/renderer/resources/extensions/automation_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/automation_custom_bindings.js
@@ -4,7 +4,7 @@
 
 // Custom bindings for the automation API.
 var AutomationNode = require('automationNode').AutomationNode;
-var AutomationTree = require('automationTree').AutomationTree;
+var AutomationRootNode = require('automationNode').AutomationRootNode;
 var automation = require('binding').Binding.create('automation');
 var automationInternal =
     require('binding').Binding.create('automationInternal').generate();
@@ -16,20 +16,20 @@ var schema =
     requireNative('automationInternal').GetSchemaAdditions();
 
 // TODO(aboxhall): Look into using WeakMap
-var idToAutomationTree = {};
+var idToAutomationRootNode = {};
 var idToCallback = {};
 
 // TODO(dtseng): Move out to automation/automation_util.js or as a static member
-// of AutomationTree to keep this file clean.
+// of AutomationRootNode to keep this file clean.
 /*
- * Creates an id associated with a particular AutomationTree based upon a
+ * Creates an id associated with a particular AutomationRootNode based upon a
  * renderer/renderer host pair's process and routing id.
  */
-var createAutomationTreeID = function(pid, rid) {
+var createAutomationRootNodeID = function(pid, rid) {
   return pid + '_' + rid;
 };
 
-var DESKTOP_TREE_ID = createAutomationTreeID(0, 0);
+var DESKTOP_TREE_ID = createAutomationRootNodeID(0, 0);
 
 automation.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
@@ -38,7 +38,7 @@ automation.registerCustomHook(function(bindingsAPI) {
   apiFunctions.setHandleRequest('getTree', function getTree(tabId, callback) {
     // enableTab() ensures the renderer for the active or specified tab has
     // accessibility enabled, and fetches its process and routing ids to use as
-    // a key in the idToAutomationTree map. The callback to enableActiveTab is
+    // a key in the idToAutomationRootNode map. The callback to enableTab is is
     // bound to the callback passed in to getTree(), so that once the tree is
     // available (either due to having been cached earlier, or after an
     // accessibility event occurs which causes the tree to be populated), the
@@ -48,8 +48,8 @@ automation.registerCustomHook(function(bindingsAPI) {
         callback();
         return;
       }
-      var id = createAutomationTreeID(pid, rid);
-      var targetTree = idToAutomationTree[id];
+      var id = createAutomationRootNodeID(pid, rid);
+      var targetTree = idToAutomationRootNode[id];
       if (!targetTree) {
         // If we haven't cached the tree, hold the callback until the tree is
         // populated by the initial onAccessibilityEvent call.
@@ -65,7 +65,7 @@ automation.registerCustomHook(function(bindingsAPI) {
 
   var desktopTree = null;
   apiFunctions.setHandleRequest('getDesktop', function(callback) {
-    desktopTree = idToAutomationTree[DESKTOP_TREE_ID];
+    desktopTree = idToAutomationRootNode[DESKTOP_TREE_ID];
     if (!desktopTree) {
       if (DESKTOP_TREE_ID in idToCallback)
         idToCallback[DESKTOP_TREE_ID].push(callback);
@@ -76,7 +76,7 @@ automation.registerCustomHook(function(bindingsAPI) {
       // scope.
       automationInternal.enableDesktop(function() {
         if (lastError.hasError(chrome)) {
-          delete idToAutomationTree[DESKTOP_TREE_ID];
+          delete idToAutomationRootNode[DESKTOP_TREE_ID];
           callback();
           return;
         }
@@ -93,14 +93,14 @@ automation.registerCustomHook(function(bindingsAPI) {
 automationInternal.onAccessibilityEvent.addListener(function(data) {
   var pid = data.processID;
   var rid = data.routingID;
-  var id = createAutomationTreeID(pid, rid);
-  var targetTree = idToAutomationTree[id];
+  var id = createAutomationRootNodeID(pid, rid);
+  var targetTree = idToAutomationRootNode[id];
   if (!targetTree) {
     // If this is the first time we've gotten data for this tree, it will
     // contain all of the tree's data, so create a new tree which will be
     // bootstrapped from |data|.
-    targetTree = new AutomationTree(pid, rid);
-    idToAutomationTree[id] = targetTree;
+    targetTree = new AutomationRootNode(pid, rid);
+    idToAutomationRootNode[id] = targetTree;
   }
   privates(targetTree).impl.update(data);
   var eventType = data.eventType;
diff --git a/chrome/renderer/resources/renderer_resources.grd b/chrome/renderer/resources/renderer_resources.grd
index a419012..c0d2741 100644
--- a/chrome/renderer/resources/renderer_resources.grd
+++ b/chrome/renderer/resources/renderer_resources.grd
@@ -37,7 +37,6 @@
         <include name="IDR_AUTOMATION_CUSTOM_BINDINGS_JS" file="extensions\automation_custom_bindings.js" type="BINDATA" />
         <include name="IDR_AUTOMATION_EVENT_JS" file="extensions\automation\automation_event.js" type="BINDATA" />
         <include name="IDR_AUTOMATION_NODE_JS" file="extensions\automation\automation_node.js" type="BINDATA" />
-        <include name="IDR_AUTOMATION_TREE_JS" file="extensions\automation\automation_tree.js" type="BINDATA" />
         <include name="IDR_BROWSER_ACTION_CUSTOM_BINDINGS_JS" file="extensions\browser_action_custom_bindings.js" type="BINDATA" />
         <include name="IDR_CAST_STREAMING_RTP_STREAM_CUSTOM_BINDINGS_JS" file="extensions\cast_streaming_rtp_stream_custom_bindings.js" type="BINDATA" />
         <include name="IDR_CAST_STREAMING_SESSION_CUSTOM_BINDINGS_JS" file="extensions\cast_streaming_session_custom_bindings.js" type="BINDATA" />
diff --git a/chrome/test/data/extensions/api_test/automation/tests/desktop/actions.js b/chrome/test/data/extensions/api_test/automation/tests/desktop/actions.js
index eb27ef4..c50f3f3 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/desktop/actions.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/desktop/actions.js
@@ -4,7 +4,7 @@
 
 var allTests = [
   function testDoDefault() {
-    var firstTextField = findAutomationNode(tree.root,
+    var firstTextField = findAutomationNode(rootNode,
         function(node) {
           return node.role == 'textField';
         });
@@ -16,7 +16,7 @@ var allTests = [
   },
 
   function testFocus() {
-    var firstFocusableNode = findAutomationNode(tree.root,
+    var firstFocusableNode = findAutomationNode(rootNode,
         function(node) {
           return node.role == 'button' && node.state.focusable;
         });
diff --git a/chrome/test/data/extensions/api_test/automation/tests/desktop/common.js b/chrome/test/data/extensions/api_test/automation/tests/desktop/common.js
index 1887fc8..61d5a6b 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/desktop/common.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/desktop/common.js
@@ -10,7 +10,7 @@ var EventType = chrome.automation.EventType;
 var RoleType = chrome.automation.RoleType;
 var StateType = chrome.automation.StateType;
 
-var tree = null;
+var rootNode = null;
 
 function findAutomationNode(root, condition) {
   if (condition(root))
@@ -26,8 +26,8 @@ function findAutomationNode(root, condition) {
 }
 
 function setupAndRunTests(allTests) {
-  chrome.automation.getDesktop(function(treeArg) {
-    tree = treeArg;
+  chrome.automation.getDesktop(function(rootNodeArg) {
+    rootNode = rootNodeArg;
     chrome.test.runTests(allTests);
   });
 }
diff --git a/chrome/test/data/extensions/api_test/automation/tests/desktop/desktop.js b/chrome/test/data/extensions/api_test/automation/tests/desktop/desktop.js
index 3ea84c4..12a2297 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/desktop/desktop.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/desktop/desktop.js
@@ -4,30 +4,30 @@
 
 var allTests = [
   function testGetDesktop() {
-    chrome.automation.getDesktop(function(tree) {
-      assertEq(RoleType.desktop, tree.root.role);
-      assertEq(RoleType.window, tree.root.firstChild().role);
+    chrome.automation.getDesktop(function(rootNode) {
+      assertEq(RoleType.desktop, rootNode.role);
+      assertEq(RoleType.window, rootNode.firstChild().role);
       chrome.test.succeed();
     });
   },
 
   function testGetDesktopTwice() {
     var desktop = null;
-    chrome.automation.getDesktop(function(tree) {
-      desktop = tree;
+    chrome.automation.getDesktop(function(rootNode) {
+      desktop = rootNode;
     });
-    chrome.automation.getDesktop(function(tree) {
-      assertEq(tree, desktop);
+    chrome.automation.getDesktop(function(rootNode) {
+      assertEq(rootNode, desktop);
       chrome.test.succeed();
     });
   },
 
   function testGetDesktopNested() {
     var desktop = null;
-    chrome.automation.getDesktop(function(tree) {
-      desktop = tree;
-      chrome.automation.getDesktop(function(tree2) {
-        assertEq(tree2, desktop);
+    chrome.automation.getDesktop(function(rootNode) {
+      desktop = rootNode;
+      chrome.automation.getDesktop(function(rootNode2) {
+        assertEq(rootNode2, desktop);
         chrome.test.succeed();
       });
     });
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/actions.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/actions.js
index 911c988..e9387b9 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/actions.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/actions.js
@@ -4,7 +4,7 @@
 
 var allTests = [
   function testSimpleAction() {
-    var okButton = tree.root.firstChild().firstChild();
+    var okButton = rootNode.firstChild().firstChild();
     okButton.addEventListener(EventType.focus, function() {
       chrome.test.succeed();
     }, true);
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/common.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/common.js
index 753899f..b794e0cf 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/common.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/common.js
@@ -10,7 +10,7 @@ var EventType = chrome.automation.EventType;
 var RoleType = chrome.automation.RoleType;
 var StateType = chrome.automation.StateType;
 
-var tree = null;
+var rootNode = null;
 
 function createTab(url, callback) {
   chrome.tabs.create({"url": url}, function(tab) {
@@ -21,9 +21,9 @@ function createTab(url, callback) {
 function setUpAndRunTests(allTests) {
   getUrlFromConfig(function(url) {
     createTab(url, function(unused_tab) {
-      chrome.automation.getTree(function (returnedTree) {
-        tree = returnedTree;
-        tree.addEventListener('loadComplete', function() {
+      chrome.automation.getTree(function (returnedRootNode) {
+        rootNode = returnedRootNode;
+        rootNode.addEventListener('loadComplete', function() {
           chrome.test.runTests(allTests);
         });
       });
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/events.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/events.js
index 065d03c..f354a94 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/events.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/events.js
@@ -4,7 +4,7 @@
 
 var allTests = [
   function testEventListenerTarget() {
-    var cancelButton = tree.root.firstChild().children()[2];
+    var cancelButton = rootNode.firstChild().children()[2];
     assertEq('Cancel', cancelButton.attributes.name);
     cancelButton.addEventListener(EventType.focus,
                                   function onFocusTarget(event) {
@@ -16,7 +16,7 @@ var allTests = [
     cancelButton.focus();
   },
   function testEventListenerBubble() {
-    var cancelButton = tree.root.firstChild().children()[2];
+    var cancelButton = rootNode.firstChild().children()[2];
     assertEq('Cancel', cancelButton.attributes.name);
     var cancelButtonGotEvent = false;
     cancelButton.addEventListener(EventType.focus,
@@ -24,21 +24,21 @@ var allTests = [
       cancelButtonGotEvent = true;
       cancelButton.removeEventListener(EventType.focus, onFocusBubble);
     });
-    tree.root.addEventListener(EventType.focus,
+    rootNode.addEventListener(EventType.focus,
                                function onFocusBubbleRoot(event) {
       assertEq('focus', event.type);
       assertEq(cancelButton, event.target);
       assertTrue(cancelButtonGotEvent);
-      tree.root.removeEventListener(EventType.focus, onFocusBubbleRoot);
+      rootNode.removeEventListener(EventType.focus, onFocusBubbleRoot);
       chrome.test.succeed();
     });
     cancelButton.focus();
   },
   function testStopPropagation() {
-    var cancelButton = tree.root.firstChild().children()[2];
+    var cancelButton = rootNode.firstChild().children()[2];
     assertEq('Cancel', cancelButton.attributes.name);
     function onFocusStopPropRoot(event) {
-      tree.root.removeEventListener(EventType.focus, onFocusStopPropRoot);
+      rootNode.removeEventListener(EventType.focus, onFocusStopPropRoot);
       chrome.test.fail("Focus event was propagated to root");
     };
     cancelButton.addEventListener(EventType.focus,
@@ -46,15 +46,15 @@ var allTests = [
       cancelButton.removeEventListener(EventType.focus, onFocusStopProp);
       event.stopPropagation();
       window.setTimeout((function() {
-        tree.root.removeEventListener(EventType.focus, onFocusStopPropRoot);
+        rootNode.removeEventListener(EventType.focus, onFocusStopPropRoot);
         chrome.test.succeed();
       }).bind(this), 0);
     });
-    tree.root.addEventListener(EventType.focus, onFocusStopPropRoot);
+    rootNode.addEventListener(EventType.focus, onFocusStopPropRoot);
     cancelButton.focus();
   },
   function testEventListenerCapture() {
-    var cancelButton = tree.root.firstChild().children()[2];
+    var cancelButton = rootNode.firstChild().children()[2];
     assertEq('Cancel', cancelButton.attributes.name);
     var cancelButtonGotEvent = false;
     function onFocusCapture(event) {
@@ -63,14 +63,14 @@ var allTests = [
       chrome.test.fail("Focus event was not captured by root");
     };
     cancelButton.addEventListener(EventType.focus, onFocusCapture);
-    tree.root.addEventListener(EventType.focus,
+    rootNode.addEventListener(EventType.focus,
                                function onFocusCaptureRoot(event) {
       assertEq('focus', event.type);
       assertEq(cancelButton, event.target);
       assertFalse(cancelButtonGotEvent);
       event.stopPropagation();
-      tree.root.removeEventListener(EventType.focus, onFocusCaptureRoot);
-      tree.root.removeEventListener(EventType.focus, onFocusCapture);
+      rootNode.removeEventListener(EventType.focus, onFocusCaptureRoot);
+      rootNode.removeEventListener(EventType.focus, onFocusCapture);
       window.setTimeout(chrome.test.succeed.bind(this), 0);
     }, true);
     cancelButton.focus();
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/location.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/location.js
index da6d71c..97cb461 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/location.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/location.js
@@ -5,21 +5,21 @@
 var allTests = [
   function testLocation() {
     function assertOkButtonLocation(event) {
-      var okButton = tree.root.firstChild().firstChild();
+      var okButton = rootNode.firstChild().firstChild();
       assertTrue('location' in okButton);
       assertEq({left:100, top: 200, width: 300, height: 400},
                okButton.location);
       chrome.test.succeed();
     };
 
-    var okButton = tree.root.firstChild().firstChild();
+    var okButton = rootNode.firstChild().firstChild();
     assertTrue('location' in okButton, 'no location in okButton');
     assertTrue('left' in okButton.location, 'no left in location');
     assertTrue('top' in okButton.location, 'no top in location');
     assertTrue('height' in okButton.location, 'no height in location');
     assertTrue('width' in okButton.location, 'no width in location');
 
-    tree.root.addEventListener(
+    rootNode.addEventListener(
         EventType.childrenChanged, assertOkButtonLocation);
     chrome.tabs.executeScript({ 'code':
           'document.querySelector("button")' +
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/sanity_check.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/sanity_check.js
index 73d5d2d..4667fc9 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/sanity_check.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/sanity_check.js
@@ -12,13 +12,13 @@ var RemoveUntestedStates = function(state) {
 
 var allTests = [
   function testSimplePage() {
-    var title = tree.root.attributes.docTitle;
+    var title = rootNode.attributes.docTitle;
     assertEq('Automation Tests', title);
-    RemoveUntestedStates(tree.root.state);
+    RemoveUntestedStates(rootNode.state);
     assertEq(
       {enabled: true, focusable: true, readOnly: true},
-      tree.root.state);
-    var children = tree.root.children();
+      rootNode.state);
+    var children = rootNode.children();
     assertEq(1, children.length);
     var body = children[0];
     assertEq('body', body.attributes.htmlTag);
@@ -48,11 +48,11 @@ var allTests = [
              cancelButton.state);
 
     // Traversal.
-    assertEq(undefined, tree.root.parent());
-    assertEq(tree.root, body.parent());
+    assertEq(undefined, rootNode.parent());
+    assertEq(rootNode, body.parent());
 
-    assertEq(body, tree.root.firstChild());
-    assertEq(body, tree.root.lastChild());
+    assertEq(body, rootNode.firstChild());
+    assertEq(body, rootNode.lastChild());
 
     assertEq(okButton, body.firstChild());
     assertEq(cancelButton, body.lastChild());
@@ -73,6 +73,11 @@ var allTests = [
     assertEq(undefined, cancelButton.nextSibling());
 
     chrome.test.succeed();
+  },
+  function testIsRoot() {
+    assertTrue(rootNode.isRootNode);
+    assertFalse(rootNode.firstChild().isRootNode);
+    chrome.test.succeed();
   }
 ];
 
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js b/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js
index e1ae825..8c26df4 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js
@@ -20,9 +20,9 @@ var allTests = [
       // Keep the NTP as the active tab so that we know we're requesting the
       // tab by ID rather than just getting the active tab still.
       createBackgroundTab(url, function(tab) {
-        chrome.automation.getTree(tab.id, function(tree) {
-          tree.addEventListener('loadComplete', function() {
-            var title = tree.root.attributes['docTitle'];
+        chrome.automation.getTree(tab.id, function(rootNode) {
+          rootNode.addEventListener('loadComplete', function() {
+            var title = rootNode.attributes['docTitle'];
             chrome.test.assertEq('Automation Tests', title);
             chrome.test.succeed();
           });
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/actions.js b/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/actions.js
index 9aa9fdd..ec80375 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/actions.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/actions.js
@@ -6,7 +6,7 @@ var allTests = [
   function testError() {
     chrome.automation.getTree(function(tree) {
       try {
-        tree.root.focus();
+        rootNode.focus();
       } catch (e) {
         chrome.test.assertEq('focus requires {"desktop": true} or' +
                 ' {"interact": true} in the "automation" manifest key.',
diff --git a/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/common.js b/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/common.js
index dc9cd1d..fed7094 100644
--- a/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/common.js
+++ b/chrome/test/data/extensions/api_test/automation/tests/tabs_automation_boolean/common.js
@@ -6,7 +6,7 @@ var assertEq = chrome.test.assertEq;
 var assertFalse = chrome.test.assertFalse;
 var assertTrue = chrome.test.assertTrue;
 
-var tree = null;
+var rootNode = null;
 
 function setUpAndRunTests(allTests) {
   chrome.test.getConfig(function(config) {
@@ -14,9 +14,9 @@ function setUpAndRunTests(allTests) {
     var url = 'http://a.com:PORT/index.html'
         .replace(/PORT/, config.testServer.port);
 
-    function gotTree(returnedTree) {
-      tree = returnedTree;
-      tree.addEventListener('loadComplete', function() {
+    function gotTree(returnedRootNode) {
+      rootNode = returnedRootNode;
+      rootNode.addEventListener('loadComplete', function() {
         chrome.test.runTests(allTests);
       });
     }
diff --git a/extensions/renderer/resources/utils.js b/extensions/renderer/resources/utils.js
index dd1ec2a..853c652 100644
--- a/extensions/renderer/resources/utils.js
+++ b/extensions/renderer/resources/utils.js
@@ -72,15 +72,18 @@ function loadTypeSchema(typeName, defaultSchema) {
  * instance of the implementation class).
  * @param {string} name The name of the exposed wrapper class.
  * @param {Object} cls The class implementation.
- * @param {{functions: ?Array.<string>,
+ * @param {{superclass: ?Function,
+ *          functions: ?Array.<string>,
  *          properties: ?Array.<string>,
  *          readonly: ?Array.<string>}} exposed The names of properties on the
- *     implementation class to be exposed. |functions| represents the names of
- *     functions which should be delegated to the implementation; |properties|
- *     are gettable/settable properties and |readonly| are read-only properties.
+ *     implementation class to be exposed. |superclass| represents the
+ *     constructor of the class to be used as the superclass of the exposed
+ *     class; |functions| represents the names of functions which should be
+ *     delegated to the implementation; |properties| are gettable/settable
+ *     properties and |readonly| are read-only properties.
  */
 function expose(name, cls, exposed) {
-  var publicClass = createClassWrapper(name, cls);
+  var publicClass = createClassWrapper(name, cls, exposed.superclass);
 
   if ('functions' in exposed) {
     $Array.forEach(exposed.functions, function(func) {
diff --git a/extensions/renderer/utils_native_handler.cc b/extensions/renderer/utils_native_handler.cc
index 5c8bf5d..1c0941f 100644
--- a/extensions/renderer/utils_native_handler.cc
+++ b/extensions/renderer/utils_native_handler.cc
@@ -21,25 +21,34 @@ UtilsNativeHandler::~UtilsNativeHandler() {}
 
 void UtilsNativeHandler::CreateClassWrapper(
     const v8::FunctionCallbackInfo<v8::Value>& args) {
-  CHECK_EQ(2, args.Length());
+  CHECK_EQ(3, args.Length());
   CHECK(args[0]->IsString());
   std::string name = *v8::String::Utf8Value(args[0]);
   CHECK(args[1]->IsObject());
-  v8::Local<v8::Object> obj = args[1].As<v8::Object>();
+  v8::Local<v8::Object> cls = args[1].As<v8::Object>();
+  CHECK(args[2]->IsObject() || args[2]->IsUndefined());
+  v8::Local<v8::Value> superclass = args[2];
 
   v8::HandleScope handle_scope(GetIsolate());
   // TODO(fsamuel): Consider moving the source wrapping to ModuleSystem.
   v8::Handle<v8::String> source = v8::String::NewFromUtf8(
       GetIsolate(),
       base::StringPrintf(
-          "(function($Object, $Function, privates, cls) {"
+          "(function($Object, $Function, privates, cls, superclass) {"
           "'use strict';\n"
-          "  return function %s() {\n"
-          "  var privateObj = $Object.create(cls.prototype);\n"
-          "  $Function.apply(cls, privateObj, arguments);\n"
-          "  privateObj.wrapper = this;\n"
-          "  privates(this).impl = privateObj;\n"
-          "}})",
+          "  function %s() {\n"
+          "    var privateObj = $Object.create(cls.prototype);\n"
+          "    $Function.apply(cls, privateObj, arguments);\n"
+          "    privateObj.wrapper = this;\n"
+          "    privates(this).impl = privateObj;\n"
+          "  };\n"
+          "  if (superclass) {\n"
+          "    %s.prototype = Object.create(superclass.prototype);\n"
+          "  }\n"
+          "  return %s;\n"
+          "})",
+          name.c_str(),
+          name.c_str(),
           name.c_str()).c_str());
   v8::Handle<v8::Value> func_as_value = context()->module_system()->RunString(
       source, v8::String::NewFromUtf8(GetIsolate(), name.c_str()));
@@ -57,7 +66,8 @@ void UtilsNativeHandler::CreateClassWrapper(
       context()->safe_builtins()->GetFunction(),
       natives->Get(v8::String::NewFromUtf8(
           GetIsolate(), "privates", v8::String::kInternalizedString)),
-      obj};
+      cls,
+      superclass};
   v8::Local<v8::Value> result;
   {
     v8::TryCatch try_catch;