anchor.diff:      }
     }
   
  -  void _debugReportDuplicateIds() {
  +  bool _debugHasDuplicateIds() {
       var idSet = new HashSet<String>();
       for (var child in children) {
         if (child is Text) {
  
positive.code:   }) : this.children = children == null ? _emptyList : children,
         super(key:key) {
  
      if (_isInCheckedMode) {
        _debugReportDuplicateIds();
      }
    }
  
    void _remove() {
  
negative.code:                  of another node, they must have unique keys.''';
        }
      }
      return false;
    }
  
    void _ensureClass() {
  