dillon1000/react

Make setState in render a warning, not an invariant

We keep track of the fact that something is rendering for a bunch of
warnings. (ReactCurrentOwner.current !== null)

Once we get rid of owner for string refs, I'll convert those to something
like "isRendering" instead. The interesting part is that feature `__DEV__`
only. It is only used for warnings. Except for this case.

This means that we can get rid of the special case for the isRendering
stack on in prod.