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
Experimenting with using the observatory profiler, I noticed two pieces of
low-hanging performance fruit. This change removes about 4% of the total
profile for layouting out and painting the flex demo.
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1155043004
