Add Dart_GetMessageNotifyCallback for querying the current message notify callback

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 a3bccb69463c5abb9e4dc11217e3eefc7207d548..420689d736a4f9dacd1df9232ad153a77258d16a 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1577,6 +1577,13 @@ DART_EXPORT void Dart_SetMessageNotifyCallback(
 }
 
 
+DART_EXPORT Dart_MessageNotifyCallback Dart_GetMessageNotifyCallback() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  return isolate->message_notify_callback();
+}
+
+
 struct RunLoopData {
   Monitor* monitor;
   bool done;