Update for React 0.12

Counterpart to https://github.com/facebook/react/commit/8f1657bba618846b7a111a2caee855bf59145c5a.

We've decided on a new naming convention for ReactDescriptor. It's now
called ReactElement, which is a subset of the ReactNode union type.
> to React

This is what happens when @sebmarkbage copy+pastes things.