animal.dart

void main(List<String> arguments) {
  Animal dog = Dog();
  Animal cat = Cat();

  live(dog);
  live(cat);
}

void live(Animal animal) {
  animal.breathe();
}

abstract class Animal {
  void breathe();
}

class Dog implements Animal {
  @override
  void breathe() {
    print('Dog is breathing');
  }
}

class Cat implements Animal {
  @override
  void breathe() {
    print('Cat is breathing');
  }
}