Move ReactTextComponent creation to parent

ReactTextComponent's implementation is DOM-specific; instead of flattenChildren creating the ReactTextComponent instances, we now ask the parent component to instantiate a ReactTextComponent. I don't love adding a new method to ReactDOMComponent like this, but I think this is cleaner overall. (We could now make ReactART support raw text components if we want.)

Test Plan: jest, use ballmer-peak example.

Reviewers: @sebmarkbage @jordwalke