Add dummy dart:html library for documentation.

lib/html/doc/interface/MenuElement.dartdoc

Index: lib/html/doc/interface/MenuElement.dartdoc
diff --git a/lib/html/doc/interface/MenuElement.dartdoc b/lib/html/doc/interface/MenuElement.dartdoc
new file mode 100644
index 0000000000000000000000000000000000000000..76e06251d17a73dd26bd24ca54f6417f6f9f51b2
--- /dev/null
+++ b/lib/html/doc/interface/MenuElement.dartdoc
@@ -0,0 +1,16 @@
+// 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 HTMLMenuElement
+interface MenuElement extends Element default _Elements {
+
+  MenuElement();
+
+  /** @domName HTMLMenuElement.compact */
+  bool compact;
+}