React version: 0.0.0-f77c7b9d7

Steps To Reproduce

1. listen to onFocus
2. read event.type

Link to code example:

https://codesandbox.io/s/eventtype-in-reactnext-gt71n?file=/src/Demo.js

The current behavior

event.type === 'focusin' for focus and focusout for blur.

The expected behavior

event.type is unaffected by changes to the native event React listens to. At least onMouseEnter works that way where you (sometimes?) have a mouseover native event but event.type will still read mouseover.