Add dummy dart:html library for documentation.

lib/html/doc/interface/HeadingElement.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 HTMLHeadingElement
+interface HeadingElement extends Element default _Elements {
+
+  HeadingElement.h1();
+
+  HeadingElement.h2();
+
+  HeadingElement.h3();
+
+  HeadingElement.h4();
+
+  HeadingElement.h5();
+
+  HeadingElement.h6();
+
+  /** @domName HTMLHeadingElement.align */
+  String align;
+}