The names are probably less familiar, but more consistent:
- FrameCallback: a callback that is relative to the frame and wants the
  frame offset (a duration) as argument.
- addXFrameCallback: adds the given callback to the internal lists/maps.
- scheduleXFrameCallback (currently only X = ""): add the callback, but
  also trigger a new frame.
- handleX: the method that is invoked when the event-loop or the frame
  calls into the scheduler.
- ensureXYZ: ensure that the callback happens.
  Unfortunately there is the ambiguity between a "callback": it can be a
  closure, or the action of doing a callback, so we end up with:
  ensureBeginFrameCallback, and ensureEventLoopCallback, where
  "callback" means the action of being called back.

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

This patch folds the functionality from AnimatedSimulation into Timeline.

The only client wants a zero-based duration.

This patch prepares us to switch to using integers when handing off the
animation time from the engine to the framework.

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.

1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.