Switch to mixins for Dart 3 (dart-archive#123)

* Switch to mixins for Dart 3

* Switch to 3.0.0 for testing

* Ready to publish

update CI config; use lints/recommended (dart-archive#108)

* update CI config; use lints/recommended

* Update CHANGELOG.md

Release 0.1.7 (dart-archive#21)

Lower mockito dependency to 2.0.0 (dart-archive#19)

Mockito v2.2.0 has a SDK constraint that isn't compatible with bleeding edge Dart SDK. v2.0.0 looks fine, and we don't depend on anything new from 2.2.0 anyway.