Use metadata annotation @patch for patch classes

runtime/bin/eventhandler_patch.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.
 
 import 'dart:nativewrappers';
 
-patch class _EventHandler {
-  /* patch */ static void _sendData(Object sender,
+@patch class _EventHandler {
+  /* @patch */ static void _sendData(Object sender,
                                     SendPort sendPort,
                                     int data)
       native "EventHandler_SendData";
 
   static int _timerMillisecondClock()
       native "EventHandler_TimerMillisecondClock";
 }