Create an example app that demonstrates interactive coordination of an fn tree and a raw RenderObject tree.

Sector changes:
- implement the intrinsic sizing box API on RenderBoxToRenderSectorAdapter
- remove some debug print statements
- fix getIntrinsicDimensions() on RenderSolidColor to return true values
- factor out the default demo

RenderObject changes:
- BoxConstraints.isInfinite() now returns true only if both dimensions are infinite

fn changes:
- implement UINodeToRenderBoxAdapter
- rename RenderObjectToUINodeAdapter to RenderBoxToUINodeAdapter

Tests:
- add a test for sector layout
- make TestRenderView support being run without the unit test framework

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1175423007.