Many of the widgets that use CustomPaint were spamming repaints because
CustomPaint repaints when the identity of the onPaint callback changes, which
it does every build for StatelessComponents.
This patch changes CustomPaint to use a CustomPainter, similar to the new
custom layout widgets. The CustomPainter has a `shouldRepaint` function along
with its `paint` function. This function gives clients explicit control over
when the custom paint object repaints.
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.
