anchor.diff:    testConcurrentAddSelf([1, 2, 3]);
   }
   
  -void testConcurrentModification(List<int?> list) {
  +void testConcurrentModification(List<int> list) {
     // add, removeLast.
     list.clear();
     list.addAll([1, 2, 3, 2, 7, 9, 9, 7, 2, 3, 2, 1]);
  
positive.code: void main() {
    // Growable lists. Initial length 0.
    testConcurrentModification(<int>[].toList());
    testConcurrentModification(
        new List<int>.filled(0, null, growable: true).toList());
    testConcurrentModification(new List<int?>.filled(0, null).toList());
    testConcurrentModification(new List.filled(0, null, growable: true));
    testConcurrentModification([]);
    testConcurrentModification(new List.from(const []));
    testConcurrentModification(new MyList([]));
  
negative.code:     _source.length = length;
    }
  
    void add(E element) {
      _source.add(element);
    }
  
    E operator [](int index) => _source[index];
    void operator []=(int index, E value) {
      _source[index] = value;
  