Add dummy dart:html library for documentation.

lib/html/doc/interface/BodyElement.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 HTMLBodyElement
+interface BodyElement extends Element default _Elements {
+
+  BodyElement();
+
+  /**
+   * @domName EventTarget.addEventListener, EventTarget.removeEventListener, EventTarget.dispatchEvent
+   */
+  BodyElementEvents get on();
+
+  /** @domName HTMLBodyElement.aLink */
+  String aLink;
+
+  /** @domName HTMLBodyElement.background */
+  String background;
+
+  /** @domName HTMLBodyElement.bgColor */
+  String bgColor;
+
+  /** @domName HTMLBodyElement.link */
+  String link;
+
+  /** @domName HTMLBodyElement.vLink */
+  String vLink;
+}
+
+interface BodyElementEvents extends ElementEvents {
+
+  EventListenerList get beforeUnload();
+
+  EventListenerList get blur();
+
+  EventListenerList get error();
+
+  EventListenerList get focus();
+
+  EventListenerList get hashChange();
+
+  EventListenerList get load();
+
+  EventListenerList get message();
+
+  EventListenerList get offline();
+
+  EventListenerList get online();
+
+  EventListenerList get popState();
+
+  EventListenerList get resize();
+
+  EventListenerList get storage();
+
+  EventListenerList get unload();
+}