Eslint fixup

This builds on #2662 so we can maybe turn it on and make this actually block checkins.

I fixed up (almost) everything in https://gist.github.com/zpao/a0fe0a93a86ea2cfcef2. I didn't fix `LegacyImmutableObject.js` because I plan to delete that and the `args` stuff in `ReactClass.js` is just because eslint doesn't understand rest params.

This also doesn't fix any test files because lint didn't run on them.