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

Question

I just tried the following in DartPad:

void main() {
  int? x;
}

and get the following error:

Error compiling to JavaScript         


        

Answer 1

Add the following line to the analysis_options.yaml

analyzer:
  enable-experiment:
    - non-nullable

Answer 2

You enable the experiment by passing the flag --enable-experiment=non-nullable to the compiler or analyzer.

It is not a complete feature yet, so there are no promises about what it will do. Feel free to experiment, but don't use the flag for anything serious.

Answer 3

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
   

Answer 4

For me, removing pubspec.lock and rerun fix the issue. I was importing the project from Windows to Mac.

Answer 5

Manually removing pubspec.lock and running flutter pub get resolved the problem. In my case it was due to regression.