Add dummy dart:html library for documentation.

lib/html/doc/interface/LabelElement.dart

+// 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 HTMLLabelElement
+interface LabelElement extends Element default _Elements {
+
+  LabelElement();
+
+  /** @domName HTMLLabelElement.control */
+  final Element control;
+
+  /** @domName HTMLLabelElement.form */
+  final FormElement form;
+
+  /** @domName HTMLLabelElement.htmlFor */
+  String htmlFor;
+}