Migrate to null safety (dart-lang/test_descriptor#29)

Dropped support for archives as planned in https://github.com/dart-lang/test_descriptor/issues/25 - this also allows us to migrate before `package:archive`.

Closes https://github.com/dart-lang/test_descriptor/issues/25
15 files changed
tree: d917270475e55d13a21d7a04bd8b972cb81ccb3e
  1. pkgs/