Make Simulate.mouseEnter/Leave use direct dispatch

Fixes #1297.

onMouseEnter and onMouseLeave shouldn't *actually* use direct dispatch, but doing so is more useful than doing nothing (and I don't think it precludes adding proper enter/leave dispatching later, either).

Test Plan: grunt test
It looks like the latest code (0.12.1) uses `ReactBrowserEventEmitter` instead of `ReactEventEmitter`