Dart "protected methods" via interfaces & mixins

main.dart

abstract class MyType {
  void doStuff();
}

mixin MyTypeHelpers {
  void protectedUtilityMethod() => print('Protected method');
}

abstract class MyDerivedType implements MyType {
  factory MyDerivedType.create() => MyDerivedTypeImpl();
  void doMoreStuff();
}

class MyDerivedTypeImpl with MyTypeHelpers implements MyDerivedType {
  void doStuff() {
    print('Doing stuff');
    protectedUtilityMethod();
  }

  void doMoreStuff() {
    print('Doing more stuff');
    protectedUtilityMethod();
  }
}

void main() {
  var value = MyDerivedType.create();
  value.doStuff();
  value.doMoreStuff();
  // Calling value.protectedUtilityMethod is a compile-time error
}