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 were making local copies of the listener maps, but we were actually
iterating the underlying sets. Now we make local copies of the sets.

Fixes #803

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.