anchor.diff:    Future<List<int>> getModificationTimes(List<Source> sources) async {
       return sources.map((source) {
         String path = source.fullName;
  -      return _overlayModificationStamps[path] ??
  +      return _overlays[path]?.modificationStamp ??
             baseProvider.getFile(path).modificationStamp;
       }).toList();
     }
  
positive.code:     }
      _file.writeAsStringSync(content);
    }
  }
  
  /// A folder from an [OverlayResourceProvider].
  
negative.code:   /// do not have an overlay.
    final ResourceProvider baseProvider;
  
    /// A map from the paths of files for to the overlay data.
    final Map<String, _OverlayFileData> _overlays = {};
  
    /// Initialize a newly created resource provider to represent an overlay on
    /// the given [baseProvider].
  