[nnbd_migration] Improve dartdev migrate command for the "apply" button.
[nnbd_migration] Improve dartdev migrate command for the "apply" button.

Change-Id: Ia41496401fc45cee1a00589495e0ef83d8ec6461
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138220
Commit-Queue: Mike Fairhurst <2002604e76601be549134505841fea93de80dc45@google.com>
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk