* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Delay unmounting elements until the end of the frame

This patch prepares us to move these elements around in the tree.

Assert that the widget is the T in State<T>

Support for decoding images in lists

Heroes: Pass next Performance to route builders

Rename RouteState to StateRoute

Remove the mixed_viewport demo

Mostly because we can, but hopefully it'll catch someone who
accidentally copypasted their createState method.

This is step 1 in making it possible to have hero transitions between
routes. To make it possible for a route to have an "exit" animation when
a new route has been pushed on top of it, we provide the next route's
AnimationPerformance to the build function. It's null if there is no
next route or if the next route has no performance.

...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.