All translate* events should have the same shape.

- Because we cache MutableEvent prototypes for each event type, any events of the same type must have the exact same properties.
- Long-term, we need to fix that FlowTranslateComposer should throw different events from FlowComposer.
- And maybe consider if there's a way to corect the cached prototype when the same event name includes different properties.
Merge pull request #1265 from asolove/translate-events

All translate* events should have the same shape.