- Reorder the methods so they are closer to run order.
- Reindent the code to match style guide.

Surface all the constructor arguments of AnimationTiming in all its subclasses.
Remove some pointless casts.
Fix some typos.
Put constructors first in class declarations.
Remove some blank lines where they just confused the structure of the code.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

Used for AnimationPerformance with a single AnimatedValue<T> variable.

Just infer it from the sign of the velocity. That's what odeon does.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

Simplifies the code a bit.

Use those in SnackBar, Drawer, navigator instead of AnimatedContainer's
intentions.

This required some changes to AnimationPerformance to better understand
animating with forces.

SnackBar, and PopupMenu instead of custom statuses.