Add dummy dart:html library for documentation.

lib/html/doc/nodoc-src/_FactoryProviders.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.
+
+class _AudioContextFactoryProvider {
+
+  factory AudioContext() => null;
+}
+
+class _PointFactoryProvider {
+
+  factory Point(num x, num y) => null;
+}
+
+class _WebSocketFactoryProvider {
+
+  factory WebSocket(String url) => null;
+}
+
+class _TextFactoryProvider {
+  factory Text(String data) => null;
+}