Migrator: use `dart pub` to check outdated
Migrator: use `dart pub` to check outdated

Fixes #42343

This also prevents a crash if the binary-to-execute could not be found.

Change-Id: I19dd9f6dcbb41a5c8b4ef2a04a87c1fc83429120
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151327
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: 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