Remove all uses of ExecutionEnvironment.global

Closes #271.

All three of these files are DOM-specific so it should be fine to use window. (ReactEventTopLevelCallback isn't obviously DOM-specific but it calls getEventTarget which is so I think we're fine here.)

Test Plan:
grunt test, tried events in a real browser and they seemed to work still.
Merge pull request #287 from spicyj/noglobal

Remove all uses of ExecutionEnvironment.global