anchor.diff:  
     Future<List<SearchResult>> _searchReferences(Element element) async {
       List<SearchResult> results = <SearchResult>[];
  -    await _addResults(results, element, IndexRelationKind.IS_REFERENCED_BY,
  -        SearchResultKind.REFERENCE);
  +    await _addResults(results, element,
  +        {IndexRelationKind.IS_REFERENCED_BY: SearchResultKind.REFERENCE});
       return results;
     }
   
  
positive.code:       int elementId = request.findElementId(element);
        if (elementId != -1) {
          CompilationUnitElement unitElement = result.unitElement;
          List<SearchResult> fileResults = request.getRelations(
              elementId, relationKind, resultKind, unitElement);
          results.addAll(fileResults);
        }
      }
  
negative.code:   }
  
    Future<Null> _addResults(List<SearchResult> results, Element element,
        Map<IndexRelationKind, SearchResultKind> relationToResultKind) async {
      String path = element.source.fullName;
  
      // If the file with the element is not known, then the element is not used.
  