There are still many more left before rendering.dart is complete.

Now MediaQuery.of always returns a non-null value. By default, you get the
values associated with the current ui.Window.

Fixes #2894

Fixes #2662

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

We'll need this for RTL support because the RTL state will live in the widget
tree. Also, remove the `oldWidget` argument to updateRenderObject because there
aren't any clients for it.

Now that the engine lets us take the view host handle, we can actually
access the view host from Dart.

This patch sketches out the basic widgets for creating Mozart child
views. We're still missing some of the machinery, so we can't quite
create child views yet, but once we supply the ViewHost and teach the
compositor how to actually display the child scenes, we'll have
something that works.