Add dummy dart:html library for documentation.

lib/html/doc/interface/NotificationCenter.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 NotificationCenter
+interface NotificationCenter {
+
+  /** @domName NotificationCenter.checkPermission */
+  int checkPermission();
+
+  /** @domName NotificationCenter.createHTMLNotification */
+  Notification createHTMLNotification(String url);
+
+  /** @domName NotificationCenter.createNotification */
+  Notification createNotification(String iconUrl, String title, String body);
+
+  /** @domName NotificationCenter.requestPermission */
+  void requestPermission(VoidCallback callback);
+}