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.
- add debugDescribeSettings to a few classes that were missing it
- fix some minor bugs in RenderShrinkWrapWidth and ShrinkWrapWidth
- introduce RenderShrinkWrapHeight and ShrinkWrapHeight