Put target on synthetic event, leave native alone

The Object.create code in src/event/NormalizedEventListener.js was causing problems in IE -- 8 doesn't support Object.create and 9 and 10 weren't happy with the assignment to the read-only target property even after the copy using Object.create. Instead, pass the target property around separately and attach it directly to the AbstractEvent.

I'm told that the FB codebase has 38 uses of .nativeEvent.target but that they can be changed.