- 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