Add dummy dart:html library for documentation.

lib/html/doc/interface/HeadingElement.dart

Index: lib/html/doc/interface/HeadingElement.dart
diff --git a/lib/html/doc/interface/HeadingElement.dart b/lib/html/doc/interface/HeadingElement.dart
new file mode 100644
index 0000000000000000000000000000000000000000..372ab50e48395d7e077f29271277716eb8722c59
--- /dev/null
+++ b/lib/html/doc/interface/HeadingElement.dart
@@ -0,0 +1,24 @@
+// 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: Do not edit - generated code.
+
+/// @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;
+}