- force the time dilation to 1.0 for the Widget tests, so that a local
change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
the engine doesn't trigger any confusing frames after our fake frames.
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.
