Implement @patch annotation for patch class members

runtime/bin/common_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 _IOCrypto {
-  /* @patch */ static Uint8List getRandomBytes(int count)
+  @patch static Uint8List getRandomBytes(int count)
       native "Crypto_GetRandomBytes";
 }
 
 _setupHooks() {
   VMLibraryHooks.eventHandlerSendData = _EventHandler._sendData;
   VMLibraryHooks.timerMillisecondClock = _EventHandler._timerMillisecondClock;
 }