Make tool/travis.sh use `flutter format` instead of `dartfmt`
Make tool/travis.sh use `flutter format` instead of `dartfmt`

The `dartfmt` command has been replaced with `dart format`.

We currently can't quite replace `dartfmt` with `dart format` because
the fix for https://github.com/dart-lang/sdk/issues/44582 hasn't
reached the Flutter beta channel yet, so we can't rely on its exit
code.  Additionally, unlike `dartfmt --dry-run`, `dart format
--output=none` prints status to stdout even if nothing's changed, so
we can no longer rely on the output being empty either.

Luckily, `flutter format` does return a proper exit code, so use
that.

google/flutter.widgets,google/flutter.widgets