anchor.diff:      for (var user in _directUsers) {
         user.invalidate();
       }
  +    _directUsers.clear();
     }
   
     @override
  
positive.code:     // Append direct referenced cycles.
      for (var node in scc) {
        var file = node.file;
        _appendDirectlyReference(cycle, signature, file.importedFiles);
        _appendDirectlyReference(cycle, signature, file.exportedFiles);
      }
  
      // Fill the cycle with libraries.
  
negative.code:     return nodesOfFiles.putIfAbsent(file, () => new _LibraryNode(this, file));
    }
  
    void _appendDirectlyReferenced(
      LibraryCycle cycle,
      ApiSignature signature,
      List<FileState> directlyReferenced,
  