These classes now share more code and have feature parity.

Now a RenderBox is considered hit if one of its children are hit or it itself
decides that it's hit. In particular, empty space inside a flex won't be hit
because none of the children are located there and a RenderFlex doesn't
consider itself hittable.

Fixes #53
Fixes #1221

RenderView has to be a singleton for sanity during tests, otherwise they
all end up in the dirty lists and we end up pumping all of them each frame.

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.