Updated tests in dev, examples/api, and tests/widgets to ensure that
they continue to pass when the default for `ThemeData.useMaterial3` is
changed to true.
This is the final set of changes required for
https://github.com/flutter/flutter/issues/127064.
Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.
