Use metadata annotation @patch for patch classes

runtime/lib/async_patch.dart

 // Copyright (c) 2015, 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:_internal";
 
 // We need to pass the value as first argument and leave the second and third
 // arguments empty (used for error handling).
 // See vm/ast_transformer.cc for usage.
 Function _asyncThenWrapperHelper(continuation) {
@@ skipping 170 matching lines @@
       // Cancellation does not affect an async generator that is
       // suspended at an await.
       if (isSuspendedAtYield) {
         scheduleGenerator();
       }
     }
     return cancellationCompleter.future;
   }
 }
 
-patch void _rethrow(Object error, StackTrace stackTrace) native "Async_rethrow";
+@patch void _rethrow(Object error, StackTrace stackTrace) native "Async_rethrow";