Improve TypeError messages from failed array conversions.

Source/bindings/v8/custom/V8MessageEventCustom.cpp

 /*
  * Copyright (C) 2009 Google Inc. 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
@@ skipping 98 matching lines @@
 
     DOMWindow* sourceArg = 0;
     if (args[6]->IsObject()) {
         v8::Handle<v8::Object> wrapper = v8::Handle<v8::Object>::Cast(args[6]);
         v8::Handle<v8::Object> window = wrapper->FindInstanceInPrototypeChain(V8Window::GetTemplate(args.GetIsolate(), worldTypeInMainThread(args.GetIsolate())));
         if (!window.IsEmpty())
             sourceArg = V8Window::toNative(window);
     }
     OwnPtr<MessagePortArray> portArray;
 
-    if (!isUndefinedOrNull(args[7])) {
+    const int portArrayIndex = 7;
+    if (!isUndefinedOrNull(args[portArrayIndex])) {
         portArray = adoptPtr(new MessagePortArray);
-        if (!getMessagePortArray(args[7], *portArray, args.GetIsolate()))
+        if (!getMessagePortArray(args[portArrayIndex], portArrayIndex + 1, *portArray, args.GetIsolate()))
             return;
     }
     event->initMessageEvent(typeArg, canBubbleArg, cancelableArg, originArg, lastEventIdArg, sourceArg, portArray.release());
 
     if (!dataArg.IsEmpty()) {
         args.Holder()->SetHiddenValue(V8HiddenPropertyName::data(args.GetIsolate()), dataArg);
         if (isolatedWorldForIsolate(args.GetIsolate()))
             event->setSerializedData(SerializedScriptValue::createAndSwallowExceptions(dataArg, args.GetIsolate()));
     }
 }
 
 void V8MessageEvent::webkitInitMessageEventMethodCustom(const v8::FunctionCallbackInfo<v8::Value>& args)
 {
     initMessageEventMethodCustom(args);
 }
 
 
 } // namespace WebCore