Summary: Views that can contain other Views need to have their ViewManager to extend ViewGroupManager. Otherwise, it may not remove child Views correctly when a parent View is being detached. This diff is changing FlatViewManager that create FlatViewGroups (that can have child Views) to extend from ViewGroupManager.
Reviewed By: ahmedre
Differential Revision: D2768667
Summary: @public This diff adds a `FlatShadowNode.forceMountToView()` method that will render its contents in it own `View`.
Reviewed By: sriramramani
Differential Revision: D2564502
Summary: @public Initial version of FlatUIImplementation lacks any primitives support (such as RCTText, RCTImageView or RCTView). This diff add the first part, RCTText (alongside with RCTVirtualText and RCTRawText).
Reviewed By: sriramramani
Differential Revision: D2693348
