Use metadata annotation @patch for patch classes

runtime/lib/timer_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.
 
-patch class Timer {
-  /*patch*/ static Timer _createTimer(Duration duration, void callback()) {
+@patch class Timer {
+  /*@patch*/ static Timer _createTimer(Duration duration, void callback()) {
     // TODO(iposva): Remove _TimerFactory and use VMLibraryHooks exclusively.
     if (_TimerFactory._factory == null) {
       _TimerFactory._factory = VMLibraryHooks.timerFactory;
     }
     if (_TimerFactory._factory == null) {
       throw new UnsupportedError("Timer interface not supported.");
     }
     int milliseconds = duration.inMilliseconds;
     if (milliseconds < 0) milliseconds = 0;
     return _TimerFactory._factory(milliseconds, (_) { callback(); }, false);
   }
 
-  /*patch*/ static Timer _createPeriodicTimer(Duration duration,
+  /*@patch*/ static Timer _createPeriodicTimer(Duration duration,
                                               void callback(Timer timer)) {
     // TODO(iposva): Remove _TimerFactory and use VMLibraryHooks exclusively.
     if (_TimerFactory._factory == null) {
       _TimerFactory._factory = VMLibraryHooks.timerFactory;
     }
     if (_TimerFactory._factory == null) {
       throw new UnsupportedError("Timer interface not supported.");
     }
     int milliseconds = duration.inMilliseconds;
     if (milliseconds < 0) milliseconds = 0;
     return _TimerFactory._factory(milliseconds, callback, true);
   }
 }
 
 typedef Timer _TimerFactoryClosure(int milliseconds,
                                    void callback(Timer timer),
                                    bool repeating);
 
 // Warning: Dartium sets _TimerFactory._factory instead of setting things up
 // through VMLibraryHooks.timerFactory.
 class _TimerFactory {
   static _TimerFactoryClosure _factory;
 }