anchor.diff:  void copyDirectory(
     Directory srcDir,
     Directory destDir, {
  -  bool Function(File srcFile, File destFile) shouldCopyFile,
  -  void Function(File srcFile, File destFile) onFileCopied,
  +  bool Function(File srcFile, File destFile)? shouldCopyFile,
  +  void Function(File srcFile, File destFile)? onFileCopied,
   }) {
     if (!srcDir.existsSync()) {
       throw Exception('Source directory "${srcDir.path}" does not exist, nothing to copy');
  
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: // Copyright 2014 The Flutter Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  import 'package:file/file.dart';
  import 'package:file/local.dart' as local_fs;
  