A MessageObject is a purely internal object to hold information about

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 440bde87e91932709181fcceed61bbbcb423ec4b..48f32447777f3f1bb283b81db52bc0cf2559144a 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -93,11 +93,12 @@ class MessageHandler {
   static void ReportMessage(const char* msg);
 
   // Returns a message object for the API to use.
-  static Handle<Object> MakeMessageObject(const char* type,
-                                          MessageLocation* loc,
-                                          Vector< Handle<Object> > args,
-                                          Handle<String> stack_trace,
-                                          Handle<JSArray> stack_frames);
+  static Handle<JSMessageObject> MakeMessageObject(
+      const char* type,
+      MessageLocation* loc,
+      Vector< Handle<Object> > args,
+      Handle<String> stack_trace,
+      Handle<JSArray> stack_frames);
 
   // Report a formatted message (needs JS allocation).
   static void ReportMessage(MessageLocation* loc, Handle<Object> message);