ReactJS 16 Lifecycle Methods + JS-Backed Maps

The 3.1.0 release will add Dart js-interop support for ReactJS 16 lifecycle methods by-way-of new UiComponent2 / UiStatefulComponent2 base classes. UiComponent / UiStatefulComponent will be deprecated in this release. It will also use JS-Backed maps for props / state, which will improve performance, simplify component debugging, and make using the ReactJS developer tools much more pleasant. Support for the Dart 1 SDK will also be dropped at this time.