Port first sector demo to fn3

Also, fix warnings in rendering/sector_layout.dart
Also, fix hit testing in rendering/sector_layout.dart
Also, add WidgetToRenderBoxAdapter
Also, make the rendering library debugging tools more resilient to
dumping stuff before layout is complete.
8 files changed
tree: 1707526ae908d3988dc77bc2ce2cc2e0b0e13414
  1. examples/
  2. packages/