Expose ReactCoroutine on __SECRET as unstable API

[jamiebuilds]

Opening PR for the sake of discussion after talking to @gaearon.

I've wanted to experiment with ReactCoroutine for awhile. I can copy the code into another project to try it out and I will, but Dan brought up adding it to __SECRET... so here's a PR

[jamiebuilds]

Note: I didn't test or think about the actual contents of this change that much, I can fix it up if the team wants to do this.

[gaearon]

I think we could move it to react package since that's the right place for "isomorphic" stuff. I think we planned to move it there after it's more fleshed out, but might as well do now.

Then export it from packages/react/src/React.js in the secret field (there's already a few things there).

Existing imports of ReactCoroutine in the reconciler would then need to be changed to read it from that internal field of require('react'). There shouldn't be many.

Does this make sense?

[jamiebuilds]

What should be done with the types being imported from ReactTypes?

Does it make sense to move the ReactTypes module into react as well?

[gaearon]

I think those should be moved to shared/. I think I did that in #11303 but it's not ready yet.

[gaearon]

Thanks for PR and the discussion. I went ahead and made a separate package for this. #11364

It will be out together with 16.1.