Add presubmit to dart_ci repository
Add presubmit to dart_ci repository

The presubmit checks that all changed files are formatted with dartfmt.

Change-Id: I8f60091fb438b2882d6e78e2a92710861545e957
Reviewed-on: https://dart-review.googlesource.com/c/dart_ci/+/144420
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/dart_ci,dart-lang/dart_ci,dart-lang/dart_ci,dart-lang/dart_ci