Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

Also, I've organized the tests by the render object they're testing.