- Address review comments from https://codereview.chromium.org/474913004/

runtime/vm/message_handler.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/message_handler.h"
 
 #include "vm/dart.h"
 #include "vm/lockers.h"
 #include "vm/port.h"
 
 namespace dart {
 
 DECLARE_FLAG(bool, trace_isolates);
 DECLARE_FLAG(bool, trace_service_pause_events);
 
 class MessageHandlerTask : public ThreadPool::Task {
  public:
   explicit MessageHandlerTask(MessageHandler* handler)
       : handler_(handler) {
     ASSERT(handler != NULL);
   }
 
-  void Run() {
+  virtual void Run() {
     handler_->TaskCallback();
   }
 
  private:
   MessageHandler* handler_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageHandlerTask);
 };
 
 
@@ skipping 285 matching lines @@
 
 void MessageHandler::decrement_control_ports() {
   MonitorLocker ml(&monitor_);
 #if defined(DEBUG)
   CheckAccess();
 #endif
   control_ports_--;
 }
 
 }  // namespace dart