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
- makes the event logic not involve a boolean return value (since we ignored it anyway)
- splits the event handling logic into two steps, hit testing and event dispatch
- introduces an App class on the Dart side to factor out the interaction with the C++ side
- ports sector-layout and simple_render_tree to the new App infrastructure
- port simple_render_tree to the new event handling logic
- implement hit testing for the sector-layout demo
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1143343004
