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

runtime/vm/message_handler.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_HANDLER_H_
 #define VM_MESSAGE_HANDLER_H_
 
 #include "vm/isolate.h"
 #include "vm/message.h"
 #include "vm/thread.h"
@@ skipping 36 matching lines @@
   //
   // Returns true on success.
   bool HandleNextMessage();
 
   // Handles any OOB messages for this message handler.  Can be used
   // even if the message handler is running on the thread pool.
   //
   // Returns true on success.
   bool HandleOOBMessages();
 
+  // Returns true if there are pending OOB messages for this message
+  // handler.
+  bool HasOOBMessages();
+
   // A message handler tracks how many live ports it has.
   bool HasLivePorts() const { return live_ports_ > 0; }
 
   intptr_t live_ports() const {
     return live_ports_;
   }
 
   bool paused() const { return paused_ > 0; }
 
   void increment_paused() { paused_++; }
@@ skipping 100 matching lines @@
   StartCallback start_callback_;
   EndCallback end_callback_;
   CallbackData callback_data_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageHandler);
 };
 
 }  // namespace dart
 
 #endif  // VM_MESSAGE_HANDLER_H_