super_in_extension

The 'super' keyword can't be used in an extension because an extension doesn't have a superclass.

The analyzer produces this diagnostic when a member declared inside an extension uses the super keyword . Extensions aren't classes and don't have superclasses, so the super keyword serves no purpose.

The following code produces this diagnostic because super can't be used in an extension:

extension E on Object {
  String get displayString => super.toString();
}

Remove the super keyword :

extension E on Object {
  String get displayString => toString();
}