Non-nullable by default: how to enable the experiment?

Enable the non-nullable experiment with three steps:

1. Add dart-sdk version into pubspec.yaml

   environment:
     sdk: '>=2.8.0-dev.0 <2.8.0'
   

2. Add enable non-nullable experiment into analysis_options.yaml

   analyzer:
     enable-experiment:
       - non-nullable
   

3. Run your Dart code:

   dart --enable-experiment=non-nullable ./bin/hello_dart.dart
   

Add the following line to the analysis_options.yaml

analyzer:
  enable-experiment:
    - non-nullable