anchor.diff:    ItemRecord _head, _tail;
   
     /**
  -   * Add the [record] before the [insertBefore] in the list of duplicates or at the end of the list
  -   * when no [insertBefore] is specified.
  +   * Append the [record] to the list of duplicates.
      *
      * Note: by design all records in the list of duplicates hold the save value in [record.item].
      */
  -  void add(ItemRecord record, ItemRecord insertBefore) {
  -    assert(insertBefore == null || insertBefore.item == record.item);
  +  void add(ItemRecord record) {
       if (_head == null) {
  -      /// pushing the first [ItemRecord] to the list
  -      assert(insertBefore == null);
         _head = _tail = record;
         record._nextDup = null;
         record._prevDup = null;
       } else {
  -      // adding a duplicate [ItemRecord] to the list
         assert(record.item ==  _head.item ||
                record.item is num && record.item.isNaN && _head.item is num && _head.item.isNaN);
  -      if (insertBefore == null) {
  -        _tail._nextDup = record;
  -        record._prevDup = _tail;
  -        record._nextDup = null;
  -        _tail = record;
  -      } else {
  -        var prev = insertBefore._prevDup;
  -        var next = insertBefore;
  -        record._prevDup = prev;
  -        record._nextDup = next;
  -        if (prev == null) {
  -          _head = record;
  -        } else {
  -          prev._nextDup = record;
  -        }
  -        next._prevDup = record;
  -      }
  +      _tail._nextDup = record;
  +      record._prevDup = _tail;
  +      record._nextDup = null;
  +      _tail = record;
       }
     }
   
  
positive.code:   }
  
    /// Required to handle num.NAN as a Map value
    dynamic _getKey(value) => value is num && value.isNaN ? nanKey : value;
  
    String toString() => "DuplicateMap($map)";
  }
  
negative.code:  * The list of duplicates is implemented by [_DuplicateItemRecordList].
   */
  class DuplicateMap {
    static final _nanKey = const Object();
    final map = new HashMap<dynamic, _DuplicateItemRecordList>();
  
    void put(ItemRecord record) {
      var key = _getKey(record.item);
      _DuplicateItemRecordList duplicates = map[key];
      if (duplicates == null) {
        duplicates = map[key] = new _DuplicateItemRecordList();
      }
      duplicates.add(record);
    }
  
    /**
  