This CL cleans up the sky/framework/animation as follows:
1) I've moved code that's used only by the custom elements framework into
sky/framework/elements/animation. This code is based on AnimationDelegates
rather than Streams.
2) Rename ScrollCurve to ScrollBehavior because it encapsulates more behavior
than just a curve.
3) Make the Generator interface explicit and mark subclasses as actual
subclasses.
4) Move Simulation into generators.dart because it implements the Generator
interface.
5) Move Animation out of generators.dart because it does not implement the
Generator interface.
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1001373002
