Mix-in sample in Dart. I want to make Seq looks like Seq in Scala...

operator_to_list_by_mixin.dart

import 'package:unittest/unittest.dart';

void main() {

  print("intersect");
  (new Seq([1, 2, 3]) & new Seq([2, 3, 4])).forEach(print);
  
  print("union");
  (new Seq([1, 2, 3]) | new Seq([2, 3, 4])).forEach(print);
  
  /*
  (you will get the following in your console)
  intersect
  2
  3
  union
  1
  2
  3
  4
  */
}

abstract class Operatable<E> implements List<E> {
  
  Operatable<E> newInstance();
  
  Operatable<E> operator &(Operatable<E> other) {
    Operatable<E> result = newInstance();
    result.addAll(this.where((e) => other.contains(e)));
    return result;
  }
  
  Operatable<E> operator |(Operatable<E> other) {
    // Terrible...Any idea?
    Set<E> set = new Set<E>();
    set.addAll(this);
    set.addAll(other);
    Operatable<E> result = newInstance();
    result.addAll(set);
    return result;
  }
}

class Seq<E> extends ListBase<E> with Operatable<E> {

  List<E> list;
  
  Seq([this.list]);
  
  int get length => list.length;
  
  void set length(int newLength) {
    list.length = newLength;
  }
  
  Iterator<E> get iterator => list.iterator;
  
  E operator [](int index) => list[index];
  
  operator []=(int index, E value) {
    list[index] = value;
  }
  
  Operatable<E> newInstance() {
    return new Seq(<E>[]);
  }
}