This patch prepares us to pass a BuildContext to RenderObjectWidgets, which
will make it possible to rebuild RenderObjectElements:
* Delay creation of the render object until mount(). That will let us pass
`this` to createRenderObject and have the inherited elements be initialized.
* Cleanup widgets that take builder closures to prepare for their
RenderObjectElement to be rebuilt more often.
* Add a test for the interaction between inherited widgets and
MixedViewport.
Related to #2598
0327141c37