The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

Previously the widgets layer only provided a concrete binding, which
makes it awkward to extend it compared to other bindings. This moves
widgets to the same style as the other layers.

In a subsequent patch I'll use this to make the tests layer saner.

Adds BuildOwner to manage the dirty list and build processing for
widgets/elements, and adds a widget unit test to make sure separation
is enforced.

Fixes #2723

This package contains WidgetTester, which is very useful when writing tests for
widgets.

Running forever is useful for profiling but makes these scripts less useful for
measurement.  :)

 - animation_bench.dart. This benchmark measures the full main-thread pipeline
   for ticking the drawer entrance and exit animation.
 - build_bench.dart. This benchmark measures a full app rebuild when there's no
   state change.
 - layout_bench.dart. This benchmark measures a full relayout.