Fix deadlock that can occur while handling service messages at a breakpoint.

runtime/vm/message.h

 // 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.
 
 #ifndef VM_MESSAGE_H_
 #define VM_MESSAGE_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 
@@ skipping 75 matching lines @@
  public:
   MessageQueue();
   ~MessageQueue();
 
   void Enqueue(Message* msg);
 
   // Gets the next message from the message queue or NULL if no
   // message is available.  This function will not block.
   Message* Dequeue();
 
+  bool IsEmpty() { return head_ == NULL; }
+
   // Clear all messages from the message queue.
   void Clear();
 
  private:
-  friend class MessageQueueTestPeer;
-
   Message* head_;
   Message* tail_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageQueue);
 };
 
 }  // namespace dart
 
 #endif  // VM_MESSAGE_H_