Rename RouteState to StateRoute

Remove the mixed_viewport demo

...for consistency with the other Route subclasses:
```
  dialog.dart:      class DialogRoute extends Route
  drag_target.dart: class DragRoute extends Route
  navigator.dart:   class PageRoute extends Route
  navigator.dart:   class StateRoute extends Route
  popup_menu.dart:  class MenuRoute extends Route
```

It's redundant with card_collection.dart, which does similar things but
much better.

This removes GlobalKey.currentElement in favour of GlobalKey.currentContext.

Fixes #1084

BoxConstraints trivial cleanup

Keep the two ConstrainedBox classes together

Add clamped simulation

Fix bounds checking error I introduced in previour cl, Address style …

Consolidate widget tests

Ensure that the item under the focal point stays in the same place despite zooming

@abarth

TBR @abarth

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

Port input_test to fn3

Make the fn3 EditableText expand to fill the width of its parent

Port dimissable_test.dart to fn3

Implement working UpdateTask in Dart

Dart code now supports the full flow that the C++ code used to: version check,
download, replace app bundle. Bonus: the Dart code is much easier to follow,
thanks to async/await!

This is part 2 of a 3-part change. The first part added new mojom
interfaces, PathService and UpdateService, to the sky_services package.

Snap scrolling: additional tests, cleanup

Simplify AnimationPerformance

s/sky-dev/flutter-dev/

AnimationPerformance had some logic for supporting multiple variables
that was hardly ever used. ValueAnimation, a subclass, has logic for
handling a single variable. I've removed the logic for handling
variables from AnimationPerformance in favour of most call sites instead
using ValueAnimation.

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

Add widgets_next.dart to help folks transition to fn3