Add dummy dart:html library for documentation.

lib/html/doc/interface/DocumentType.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 DocumentType
+interface DocumentType extends Node {
+
+  /** @domName DocumentType.entities */
+  final NamedNodeMap entities;
+
+  /** @domName DocumentType.internalSubset */
+  final String internalSubset;
+
+  /** @domName DocumentType.name */
+  final String name;
+
+  /** @domName DocumentType.notations */
+  final NamedNodeMap notations;
+
+  /** @domName DocumentType.publicId */
+  final String publicId;
+
+  /** @domName DocumentType.systemId */
+  final String systemId;
+}