Add Dart_GetMessageNotifyCallback for querying the current message notify callback

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index a614186a7c80f6d0f0adfcbc6dfb49e08dd5bb4e..abc92eb0e4ef0c2cf60b50a179809d73b41d6cc6 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1078,6 +1078,13 @@ DART_EXPORT void Dart_SetMessageNotifyCallback(
  * is impossible to mess up. */
 
 /**
+ * Query the current message notify callback for the isolate.
+ *
+ * \return The current message notify callback for the isolate.
+ */
+DART_EXPORT Dart_MessageNotifyCallback Dart_GetMessageNotifyCallback();
+
+/**
  * The VM's default message handler supports pausing an isolate before it
  * processes the first message and right after the it processes the isolate's
  * final message. This can be controlled for all isolates by two VM flags: