Abstract Classes with Constructors: Why?

abstract_class_with_constructor.dart

void main() {
  final B b1 = B();
  final C c1 = C();
}

abstract class A {
  A(){
    print('hello from A');
  }
}

class B extends A {
  // will print "hello from A"
}

class C implements A {
  // will print nothing
}