Add dummy dart:html library for documentation.

lib/html/doc/interface/MetaElement.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 HTMLMetaElement
+interface MetaElement extends Element {
+
+  /** @domName HTMLMetaElement.content */
+  String content;
+
+  /** @domName HTMLMetaElement.httpEquiv */
+  String httpEquiv;
+
+  /** @domName HTMLMetaElement.name */
+  String name;
+
+  /** @domName HTMLMetaElement.scheme */
+  String scheme;
+}