Report promise reject with no handler (behind a flag).

src/messages.cc

Index: src/messages.cc
diff --git a/src/messages.cc b/src/messages.cc
index 290f756770c7d4b287ee2f5ce6d5b2326b7fe521..cf061ee5f6b2c790a8149b23ee10a016e950b02d 100644
--- a/src/messages.cc
+++ b/src/messages.cc
@@ -34,10 +34,8 @@ void MessageHandler::DefaultMessageReport(Isolate* isolate,
 
 
 Handle<JSMessageObject> MessageHandler::MakeMessageObject(
-    Isolate* isolate,
-    const char* type,
-    MessageLocation* loc,
-    Vector< Handle<Object> > args,
+    Isolate* isolate, const char* type, MessageLocation* loc,
+    Vector<Handle<Object> > args, Handle<JSObject> promise,
     Handle<JSArray> stack_frames) {
   Factory* factory = isolate->factory();
   Handle<String> type_handle = factory->InternalizeUtf8String(type);
@@ -62,13 +60,13 @@ Handle<JSMessageObject> MessageHandler::MakeMessageObject(
       ? Handle<Object>::cast(factory->undefined_value())
       : Handle<Object>::cast(stack_frames);
 
-  Handle<JSMessageObject> message =
-      factory->NewJSMessageObject(type_handle,
-                                  arguments_handle,
-                                  start,
-                                  end,
-                                  script_handle,
-                                  stack_frames_handle);
+  Handle<Object> promise_handle =
+      promise.is_null() ? Handle<Object>::cast(factory->undefined_value())
+                        : Handle<Object>::cast(promise);
+
+  Handle<JSMessageObject> message = factory->NewJSMessageObject(
+      type_handle, arguments_handle, promise_handle, start, end, script_handle,
+      stack_frames_handle);
 
   return message;
 }