anchor.diff:    E operator [](int index) {
       if (index is! int) throw diagnoseIndexError(this, index);
       if (index >= length || index < 0) throw diagnoseIndexError(this, index);
  -    return JS('var', '#[#]', this, index);
  +    return JS('', '#[#]', this, index);
     }
   
     void operator []=(int index, E value) {
  
positive.code:     for (int i = length - 1; i >= 0; i--) {
        // TODO(22407): Improve bounds check elimination to allow this JS code to
        // be replaced by indexing.
        var element = JS('', '#[#]', this, i);
        if (test(element)) return element;
        if (length != this.length) {
          throw new ConcurrentModificationError(this);
  
negative.code:     for (int i = 1; i < length; i++) {
        // TODO(22407): Improve bounds check elimination to allow this JS code to
        // be replaced by indexing.
        var element = JS<E>('', '#[#]', this, i);
        value = combine(value, element);
        if (length != this.length) throw new ConcurrentModificationError(this);
      }
  