Abstract base class in Dart

Actually it does become simpler in dart (https://dartpad.dartlang.org/37d12fa77834c1d8a172)

// Abstract base class
abstract class Vehicle {
  final int maxSpeed;
  int speed = 0;

  Vehicle([this.maxSpeed = 0]);

  void accelerate();
  void brake();
}

// Subclass of Vehicle, the abstract baseclass
class Car extends Vehicle {
  final int doors;

  Car(int maxSpeed, this.doors) : super(maxSpeed);

  @override
  void accelerate() {
    if (speed > maxSpeed) {
      speed = maxSpeed;
      return;
    }
    speed += 2;
  }

  @override
  void brake() {
    if (speed - 2 < 0) {
      speed = 0;
      return;
    }
    this.speed -= 2;
  }
}

main() {
  // Polymorphism
  Vehicle car = new Car(180, 4);

  // Casting
  int doors = (car as Car).doors;

  // Calling abstract method
  car.accelerate();
}

I would take a look at the Language tour, there's a whole section on abstract classes

Key points:

• Abstract classes in Dart have to be marked as abstract.
• An abstract class can have "abstract methods", you just have to omit the body
• A concrete class can mark itself as "implementing" the abstract class' contract with the keyword implements. This will force you to implement all the expected behavior on the concrete class, but it won't inherit the abstract class' provided methods' implementation.
• You can extend an abstract class with the keyword extends, and the concrete class will inherit all possible behavior of the abstract class.

That's pretty much it!

Also, you might want to take a look at mixins, there's a section below the one I have you in the Language tour.