Add a warning for adding properties into ResponderSyntheticEvent

This is a additional work for facebook#5947.

src/renderers/shared/event/eventPlugins/ResponderSyntheticEvent.js

@@ -31,7 +31,7 @@ var ResponderEventInterface = {
  * @extends {SyntheticEvent}
  */
 function ResponderSyntheticEvent(dispatchConfig, dispatchMarker, nativeEvent, nativeEventTarget) {
-  SyntheticEvent.call(this, dispatchConfig, dispatchMarker, nativeEvent, nativeEventTarget);
+  return SyntheticEvent.call(this, dispatchConfig, dispatchMarker, nativeEvent, nativeEventTarget);
 }
 
 SyntheticEvent.augmentClass(ResponderSyntheticEvent, ResponderEventInterface);