We'll eventually turn this into a full fn2 component, but for now it's just an
example.

To make this work, I created a schedule.dart as a start to implementing
scheduler.md. For now, I've kept the API similar to the web platform so that
the old world can continue use it backed by sky.window.requestAnimationFrame.

R=eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1145973009