[Dart] Getter/Setter Magic

main.dart

abstract class VM {
  Host get host => _host;
  set host(Host h) {
    _host = h;
  }
  
  Host _host;
  void initialize(Host h) {
    assert(_host == null);
    
    _host = h;
    print('VM initialized');
  }
}

class VirtualBox extends VM {
  @override
  void initialize(Host h) {
    super.initialize(h);
    
    assert(host != null);
    print('VirtualBox initialized');
  }
}

class Host {
  Host() {
    print('Host startup...');
  }
  
  VM get vm => _vm;
  set vm(VM v) {
    _vm = v;
    v.initialize(this);
  }
  VM _vm;
}

void main() {
  Host machine = Host();
  machine.vm = VirtualBox();
  
  print('machine.vm is ${machine.vm.runtimeType}');
  print('vm.host is ${machine.vm.host.runtimeType}');
}