Add dummy dart:html library for documentation.

lib/html/doc/interface/NotificationCenter.dartdoc

Index: lib/html/doc/interface/NotificationCenter.dartdoc
diff --git a/lib/html/doc/interface/NotificationCenter.dartdoc b/lib/html/doc/interface/NotificationCenter.dartdoc
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+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @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);
+}