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

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
diff --git a/runtime/vm/message_handler.cc b/runtime/vm/message_handler.cc
index e5990fbbcfab92a924fa2664000b8309659a9741..1f67d6b1fd0e66b25fb6df66d0f7abaec7f7d4df 100644
--- a/runtime/vm/message_handler.cc
+++ b/runtime/vm/message_handler.cc
@@ -204,6 +204,12 @@ bool MessageHandler::HandleOOBMessages() {
 }
 
 
+bool MessageHandler::HasOOBMessages() {
+  MonitorLocker ml(&monitor_);
+  return !oob_queue_->IsEmpty();
+}
+
+
 void MessageHandler::TaskCallback() {
   ASSERT(Isolate::Current() == NULL);
   bool ok = true;