Use metadata annotation @patch for patch classes

runtime/lib/schedule_microtask_patch.dart

 // Copyright (c) 2013, 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 _AsyncRun {
-  /* patch */ static void _scheduleImmediate(void callback()) {
+@patch class _AsyncRun {
+  /* @patch */ static void _scheduleImmediate(void callback()) {
     if (_ScheduleImmediate._closure == null) {
       throw new UnsupportedError("Microtasks are not supported");
     }
     _ScheduleImmediate._closure(callback);
   }
 }
 
 typedef void _ScheduleImmediateClosure(void callback());
 
 class _ScheduleImmediate {
   static _ScheduleImmediateClosure _closure;
 }
 
 void _setScheduleImmediateClosure(_ScheduleImmediateClosure closure) {
   _ScheduleImmediate._closure = closure;
 }