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 when using public APIs.

Test Plan: jest, use ballmer-peak example.

docs/10.4-test-utils.md

@@ -75,14 +75,6 @@ boolean isCompositeComponentWithType(ReactComponent instance, function component
 
 Returns true if `instance` is a composite component (created with `React.createClass()`) whose type is of a React `componentClass`.
 
-### isTextComponent
-
-```javascript
-boolean isTextComponent(ReactComponent instance)
-```
-
-Returns true if `instance` is a plain text component.
-
 ### findAllInRenderedTree
 
 ```javascript