One might argue, the most basic utility classes.

For those times when you want to do something as you move away from a
route into the next one, as well as when you move into it from the
previous one.

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.

MixedViewport didn't use the building:true flag when locking itself, so
when it caused a rebuild of its children, we assumed that nobody was
allowed to mark things dirty below the list, and things crashed when
Inherited people did in fact rebuild.

Also:
 - default offset for MixedViewport
 - don't bother rebuilding if the underlying RenderObject is going to
   rebuild anyway for some reason
 - better docs for the "items must have keys" assert
 - keep the FlipComponent stuff together in test_widgets.dart

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

ComponentState becomes State, for brevity.
Instead of overriding its constructor, override initState().
This makes writing States much simpler.