Add dummy dart:html library for documentation.

lib/html/doc/interface/DedicatedWorkerContext.dartdoc

Index: lib/html/doc/interface/DedicatedWorkerContext.dartdoc
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+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName DedicatedWorkerContext
+interface DedicatedWorkerContext extends WorkerContext {
+
+  /**
+   * @domName EventTarget.addEventListener, EventTarget.removeEventListener, EventTarget.dispatchEvent
+   */
+  DedicatedWorkerContextEvents get on();
+
+  /** @domName DedicatedWorkerContext.postMessage */
+  void postMessage(Object message, [List messagePorts]);
+
+  /** @domName DedicatedWorkerContext.webkitPostMessage */
+  void webkitPostMessage(Object message, [List transferList]);
+}
+
+interface DedicatedWorkerContextEvents extends WorkerContextEvents {
+
+  EventListenerList get message();
+}