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

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"
 
@@ skipping 186 matching lines @@
 
 bool MessageHandler::HandleOOBMessages() {
   MonitorLocker ml(&monitor_);
 #if defined(DEBUG)
   CheckAccess();
 #endif
   return HandleMessages(false, false);
 }
 
 
+bool MessageHandler::HasOOBMessages() {
+  MonitorLocker ml(&monitor_);
+  return !oob_queue_->IsEmpty();
+}
+
+
 void MessageHandler::TaskCallback() {
   ASSERT(Isolate::Current() == NULL);
   bool ok = true;
   bool run_end_callback = false;
   {
     MonitorLocker ml(&monitor_);
     // Initialize the message handler by running its start function,
     // if we have one.  For an isolate, this will run the isolate's
     // main() function.
     if (pause_on_start()) {
@@ skipping 83 matching lines @@
 
 void MessageHandler::decrement_live_ports() {
   MonitorLocker ml(&monitor_);
 #if defined(DEBUG)
   CheckAccess();
 #endif
   live_ports_--;
 }
 
 }  // namespace dart