anchor.diff:    final Map<Uri, FileState> _uriToFile = {};
   
     /**
  +   * All known file paths.
  +   */
  +  final Set<String> knownFilePaths = new Set<String>();
  +
  +  /**
      * Mapping from a path to the corresponding [FileState]s, canonical or not.
      */
     final Map<String, List<FileState>> _pathToFiles = {};
  
positive.code:       FileSource uriSource = new FileSource(resource, uri);
        file = new FileState._(this, path, uri, uriSource);
        _uriToFile[uri] = file;
        _pathToFiles.putIfAbsent(path, () => <FileState>[]).add(file);
        _pathToCanonicalFile[path] = file;
        file.refresh();
      }
  
negative.code:       FileSource source = new FileSource(resource, uri);
        file = new FileState._(this, path, uri, source);
        _uriToFile[uri] = file;
        _addFileWithPath(path, file);
        file.refresh();
      }
      return file;
  