anchor.diff:  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(new List.filled(0, 0, growable: true));
     testConcurrentModification([]);
     testConcurrentModification(new List.from(const []));
     testConcurrentModification(new MyList([]));
  
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:     _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;
  