Finalize a *real* public API

[zpao]

In order to do a number of things, we need to be able to access modules we currently consider internal. The main need for this comes from components or other "plugins" that need to do more than usual. For example, ReactTransitionGroup accesses ReactChildren.

We don't currently want to package ReactTransitionGroup in React itself so the only option is to build it as a standlone package. We can do that but without some substantial changes (or some hacks that probably won't work everywhere), we can't do it without also packaging up React. So we need a public API where we can make this work less awkwardly.

More soon, just wanted to make sure we had this filed.

[vjeux]

This is not really actionable. Closing this one