Add dummy dart:html library for documentation.

lib/html/doc/interface/MessageChannel.dartdoc

Index: lib/html/doc/interface/MessageChannel.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 MessageChannel
+interface MessageChannel default _MessageChannelFactoryProvider {
+
+  MessageChannel();
+
+  /** @domName MessageChannel.port1 */
+  final MessagePort port1;
+
+  /** @domName MessageChannel.port2 */
+  final MessagePort port2;
+}