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

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

It turns out that an AnimatedContainer with a BoxDecoration would
start animating every single time it was built, whether or not the
container's decoration had changed.

This widget is used in Material and Drawer. We don't currently support
animating towards null, but we can add that in a future patch.