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.
This lets clients listen for when a widget with a given global key goes through
a sync operation. We'll need this for mimic to track its mimicable when it
moves around the tree.