main.dart

import 'dart:collection';

class A {
  A();
}

class B extends A {
  B() : super();
}

class Factory {
  static A build<T extends A>(int some) {
    if (some == 0) return A();
    
    return B();
  }
}


class CustomList<T extends A> with ListMixin<T> {
  CustomList();
  
  final List<A> _holder = <A>[];
  
  void populate() {
    _holder.add(Factory.build<B>(2));
  }
  
  @override
  int get length => _holder.length;

  @override
  set length(int value) {} 
  
  @override
  T operator [](int index) => _holder[index] as T;

  @override
  void operator []=(int index, T value) => throw UnimplementedError();
}


void main() {
  
  final cl = CustomList<B>();
  cl.populate();
  print(cl[0].runtimeType);
}