Add dummy dart:html library for documentation.

lib/html/doc/interface/MessageEvent.dartdoc

+// 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 MessageEvent
+interface MessageEvent extends Event {
+
+  /** @domName MessageEvent.data */
+  final Object data;
+
+  /** @domName MessageEvent.lastEventId */
+  final String lastEventId;
+
+  /** @domName MessageEvent.origin */
+  final String origin;
+
+  /** @domName MessageEvent.ports */
+  final List ports;
+
+  /** @domName MessageEvent.source */
+  final Window source;
+
+  /** @domName MessageEvent.initMessageEvent */
+  void initMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, Window sourceArg, List messagePorts);
+
+  /** @domName MessageEvent.webkitInitMessageEvent */
+  void webkitInitMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, Window sourceArg, List transferables);
+}