Flutter extension-methods not working, it says "undefined class" and "requires the extension-methods language feature"

Flutter extension-methods not working, it says "undefined class" and "requires the extension-methods language feature"

flutter dart extension-methods flutter-web class-extensions

dart

Dart extension: don't access members with 'this' unless avoiding shadowing

dart this warnings extension-methods

dart

Is there a way to add a static extension method on a class directly in Dart?

dart extension-methods

dart

Does Dart have extension methods like c#?

extension-methods dart

dart

Dart: Extension method on enum doesn't work "The method isn't defined for the class"

dart enums extension-methods

dart