To be able to use TestFairy Flutter Plugin 2.+, you must have the environment specified below and upgrade your project code base to Dart 2 using this guide.
environment:
sdk: ">=2.10.0 <3.0.0"
flutter: ">=1.22.0 <2.0.0"
Make sure your project root has an analysis_options.yaml that looks similar to this. The important part is the enabled experiment in the top declaration.