Currently the Flutter packages (e.g. flutter_test) are using Dart 2.10 SDK lower constraints. After we rolled to Dart 2.12, this triggers pub warnings:

The package resolution is not fully migrated to null-safety.

package:flutter_test is not opted into null safety in its pubspec.yaml:
  ╷
5 │   sdk: ">=2.10.0-0.0.dev <3.0.0"
  │        ^^^^^^^^^^^^^^^^^^^^^^^^^
  ╵

Either downgrade your sdk constraint, or invoke dart/flutter with
`--no-sound-null-safety`.

Issue dart-lang/sdk#44031 tracks resolving that for all Dart core packages. The present bug tracks resolving it for Flutter packages (both the ones shipped in the Flutter SDK, and the ones shipped on pub.dev).