To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

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

This makes HeroController ignore StateRoutes when deciding where to
animate heroes to and from.

- 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.

Previously, we passed widgets up the hierarchy to display them in the overlay,
but that breaks the change propagation logic because those widgets won't get
rebuilt.  Now we pass WidgetBuilders instead, which can be rebuilt when the
overlay rebuilds.

Fixes #1913

We were trying to do a hero animation from a page to itself, which doesn't make
any sense. Now we only render the "to" page offstage if it is different from
the "from" page and if its performance isn't already complete.