Use metadata annotation @patch for patch classes

runtime/lib/schedule_microtask_patch.dart

Index: runtime/lib/schedule_microtask_patch.dart
diff --git a/runtime/lib/schedule_microtask_patch.dart b/runtime/lib/schedule_microtask_patch.dart
index f08de2b907908206f03ae5fe21fd215cf64ddac4..9384c1ecb01c860d28f8ae096db5d0f8ac455c70 100644
--- a/runtime/lib/schedule_microtask_patch.dart
+++ b/runtime/lib/schedule_microtask_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 _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");
     }