And rootConstraints to configuration.

This type and its variable have nothing to do with Constraints.

Fixes https://github.com/flutter/flutter/issues/829

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

 - 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.