anchor.diff:  /// Various convenience file system methods.
   class FileSystemUtils {
     FileSystemUtils({
  -    @required FileSystem fileSystem,
  -    @required Platform platform,
  +    required FileSystem fileSystem,
  +    required Platform platform,
     }) : _fileSystem = fileSystem,
          _platform = platform;
   
  
positive.code: 
    @visibleForTesting
    LocalFileSystem.test({
      @required Signals signals,
      List<ProcessSignal> fatalSignals = Signals.defaultExitSignals,
    }) : this(signals, fatalSignals, null);
  
    Directory _systemTemp;
    final Map<ProcessSignal, Object> _signalTokens = <ProcessSignal, Object>{};
    final ShutdownHooks _shutdownHooks;
  
    Future<void> dispose() async {
      _tryToDeleteTemp();
  
negative.code:   void _tryToDeleteTemp() {
      try {
        if (_systemTemp?.existsSync() ?? false) {
          _systemTemp?.deleteSync(recursive: true);
        }
      } on FileSystemException {
        // ignore.
  