An example to demonstrate animal classification system in Dart 2.x.

AnimalClassfication.dart

class Animal { }

class Reptile extends Animal { }
class Bird extends Animal { }

mixin MobileLife {
  // We don't need to implement _move on every classes using this mixin, since mixin can include state.
  var _distance = 0.0;
  
  void _move(double displacement) {
    _distance += displacement;
    print("I moved ${_getDistance()} metres.");
  }
  
  double _getDistance() => _distance;
}

mixin Swimmer on MobileLife {
  void swim() {
    super._move(_getSwimDistance());
  }

  double _getSwimDistance() => 0.0;
}

mixin Walker on MobileLife {
  void walk() {
    super._move(_getWalkDistance());
  }

  double _getWalkDistance() => 0.0;
}

class Duck extends Bird with MobileLife, Walker, Swimmer { 
  @override
  double _getWalkDistance() => 0.05;

  @override
  double _getSwimDistance() => 0.1;
}

class Turtle extends Reptile with MobileLife, Walker, Swimmer {
  @override
  double _getSwimDistance() => 0.5;

  // Turtles are so slow to move 1 metre for 10 times of move command
  var walkTimes = 0;
  void walk() {
    walkTimes++;
    if (walkTimes % 10 == 0) {
      super._move(0.001);
    }

    super.walk();
  }
}

void main() {
  print("Duck says:");
  final duck = Duck();
  duck.walk();
  duck.swim();
  
  print("Turtle says:");
  final turtle = Turtle();
  turtle.swim();
  for(var i = 0; i < 10; i++) {
    turtle.walk();
  }
}

/*
 * Result:
 * Duck says:
 * 
 * I moved 0.05 metres.
 * I moved 0.15000000000000002 metres.
 *
 * Turtle says:
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.5 metres.
 * I moved 0.501 metres.
 * I moved 0.501 metres.
*/