Final Class

• To close the type hierarchy, use the final modifier. This prevents subTyping
  from a class outside of the current library.Disallowing both inheritance and

  implementation prevents subTyping entirely.

• You can safely add incremental change to the API.

• You can call instance methods knowing that they haven't been overwritten in a third-party subclass.

final class Vehicle {
  void moveForward(int meters) {
    // ...
  }
}

import 'a.dart';

// Can be constructed.
Vehicle myVehicle = Vehicle();

// ERROR: Can't be inherited.
class Car extends Vehicle {
  int passengers = 4;
  // ...
}

class MockVehicle implements Vehicle {
  // ERROR: Can't be implemented.
  @override
  void moveForward(int meters) {
    // ...
  }
}