main.dart

class Dog {
      String breed;
      int age; 
    
    	Dog({ this.age, this.breed });

      factory Dog.fromMap(data) {
        return Dog(
           breed: data['breed'],
        	age: data['age'],
        );
      }
    }
    
  // Map the class name to the constructor
  Map models = {
    Dog: (data) => Dog.fromMap(data)
  };
  
 // Database service 
 class Data<T> {
    T fetch() {
      Map data = { 'breed': 'pug', 'age': 10 };
      return models[T](data);
    }
  }

void main() {

  Data<Dog> dogRef = Data();
  
  Dog dog = dogRef.fetch();
	print(dog.breed);
  
}