animal.dart

abstract class Animal {
  void makeNoise();
}

class Dog extends Animal {
  @override
  void makeNoise() => print('Woof');
}

class Cat extends Animal {
  @override
  void makeNoise() => print('Meow');
}

class UtilityClass {
   void doStuff<T extends Animal>() {
    print(T.toString());
    T.makeNoise();
  }
}

void main() {
  final myClass = UtilityClass();
  myClass.doStuff<Dog>();
}