anchor.diff:    // 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: false).unit.accept(libraryBuilder);
  +  _parseString(contents, useNnbd: useNnbd).unit.accept(libraryBuilder);
     return libraryBuilder.librariesMap.sdkLibraries;
   }
   
  
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: 
    PatchFinder.parseAndVisit(String contents, {bool useNnbd})
        : contents = contents,
          unit = _parseString(contents, useNnbd: useNnbd).unit {
      visitCompilationUnit(unit);
    }
  
  