A MessageObject is a purely internal object to hold information about

src/messages.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 75 matching lines @@
 
 
 // A message handler is a convenience interface for accessing the list
 // of message listeners registered in an environment
 class MessageHandler {
  public:
   // Report a message (w/o JS heap allocation).
   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);
 
   static void DefaultMessageReport(const MessageLocation* loc,
                                    Handle<Object> message_obj);
   static Handle<String> GetMessage(Handle<Object> data);
   static SmartPointer<char> GetLocalizedMessage(Handle<Object> data);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_MESSAGES_H_