Make ReactTextComponent properly injectable

ReactTextComponent's implementation is DOM-specific; instead of flattenChildren creating the ReactTextComponent instances, ReactNativeComponent now takes care of having ReactTextComponent injected and creating the component instance. I also renamed ReactTextComponent to ReactDOMTextComponent and moved it to browser/ui/ where it belongs. ReactDOMTextComponent no longer inherits directly from ReactComponent and instead implements construct and {mount,receive,unmount}Component directly.

This diff removes `ReactTestUtils.isTextComponent` which should have previously never returned true by using public APIs.

Test Plan: jest, use ballmer-peak example.