anchor.diff:  
     PatchFinder.parseAndVisit(String contents, {bool useNnbd})
         : contents = contents,
  -        unit = _parseString(contents, useNnbd: false).unit {
  +        unit = _parseString(contents, useNnbd: useNnbd).unit {
       visitCompilationUnit(unit);
     }
   
  
positive.code:           libraryFile.lastModifiedSync().millisecondsSinceEpoch);
        var partFiles = <File>[];
        for (var part
            in _parseString(libraryContents, useNnbd: false).unit.directives) {
          if (part is PartDirective) {
            var partPath = part.uri.stringValue;
            outPaths.add(p.join(p.dirname(libraryOut), partPath));
  
negative.code:   // It doesn't understand optional new/const in Dart 2. For now, we keep
    // redundant `const` in tool/input_sdk/libraries.dart as a workaround.
    var libraryBuilder = SdkLibrariesReader_LibraryBuilder();
    _parseString(contents, useNnbd: useNnbd).unit.accept(libraryBuilder);
    return libraryBuilder.librariesMap.sdkLibraries;
  }
  
  