cond.dart

class Dog {
  String name;
  int age;

  Dog(this.name, this.age);
}

class DogHouse {
  Dog dog;
  DogHouse(this.dog);
}

void main(){
  var dog = null;
  var dogHouse = DogHouse(dog ?? Dog("Fido", 4));
  print(dogHouse.dog.name);
}