This might be helpful for #1219.

Also, remove inDebugMode since it's redundant with just using asserts,
which compile entirely out in release mode.

If the build stack got long before, it would get cropped.

These files really belong on other libraries.

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.

This patch makes it easier to debug layerization issues.