Use metadata annotation @patch for patch classes

runtime/lib/timer_patch.dart

Index: runtime/lib/timer_patch.dart
diff --git a/runtime/lib/timer_patch.dart b/runtime/lib/timer_patch.dart
index f975daa97cb926860c823c1ecf4a45f64507e1a0..a92db402f98f034c5303bbddafd7c65c0fa079b0 100644
--- a/runtime/lib/timer_patch.dart
+++ b/runtime/lib/timer_patch.dart
@@ -2,8 +2,8 @@
 // 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;
@@ -16,7 +16,7 @@ patch class Timer {
     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) {