[WIP] Use ES modules internally

[gaearon]

Based on top of https://github.com/facebook/react/pulls.

This is probably broken in some ways (and definitely breaks Jest) but some test displays Hello World. Builds are a few KBs down.

Probably don't want to land this until we have a way to run tests against bundles. Still was fun try. I intend to keep this (relatively) up to date hoping I can land it within a few weeks once the infra is there.

[acdlite]

Cool! How did you convert

if (__DEV__) {
  var warning = require('warning');
}

?

[nhunzaker]

27,000 deletions. Why such a stark drop?

[sophiebits]

It's stacked (ha) on #10517.

[nhunzaker]

Oh. Haha. @spicyj so clever!

[gaearon]

@acdlite For now I just turned everything into imports so there might be DEV-only things there. I have a list of them. We can do something like #10176 (comment) as this gets closer to reality. I think the bigger blocker is lack of test infra.

[gaearon]

Did this in another PR.