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