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.

This seems useful for numbering the entries in the list, for example.

Fixes #328

Fixes #1084

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

This argument isn't needed anymore now that State has a getter for context.

Previously, hitTestChildren in RenderBlockViewport just didn't support
horizontal scrolling, due to an oversight.