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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 1f5fb044469c778d068b8e487b5aa22f2e99b720..75e0d065119020457403883ad01e83903b0ef6f8 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1547,6 +1547,13 @@ DART_EXPORT bool Dart_HandleServiceMessages() {
 }
 
 
+DART_EXPORT bool Dart_HasServiceMessages() {
+  Isolate* isolate = Isolate::Current();
+  ASSERT(isolate);
+  return isolate->message_handler()->HasOOBMessages();
+}
+
+
 DART_EXPORT bool Dart_HasLivePorts() {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate);