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
```
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.
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.
Make it possible for named routes to be generated on the fly.
To demonstrate this, you can now long-press a stock to open it.
Next steps:
- transitions between (named) states that follow full material logic,
e.g. in the case of the stock row to stock page transition, expanding
the row into a raised sheet of material and expanding it to fit the
screen, leaving the toolbar in place but cross-fading the old
contents to the new contents.
- more information in the stock view.
While I was here I also made Material have an opinion about default text
style, so if you forget to set one, it just uses body1.
Also, fixed bugs introduced recently that made RouteState and MenuRoute
not work properly.
Introduce a Draggable class that wraps all the logic of dragging
something and dropping it on a DragTarget.
Update examples/widgets/drag_and_drop.dart accordingly.
Make the performance/transition part of routes optional.
