anchor.diff:            'This name matches one of the possible targets in libraries.json '
             'and it is used to pick which patch files will be applied.',
         allowed: ['dartdevc', 'dart2js', 'dart2js_server', 'vm', 'flutter'],
  -      defaultsTo: 'dartdevc');
  +      defaultsTo: 'dartdevc')
  +  ..addFlag('help', abbr: 'h', help: 'Display this message.');
  
positive.code: 
  void main(List<String> argv) {
    var args = _parser.parse(argv);
    String baseDir = args['out'] as String;
    if (baseDir == null) {
      var tmp = Directory.systemTemp.createTempSync('check_sdk-');
  
negative.code:     'machine',
      '--sdk-warnings',
      '--no-hints',
      '--enable-experiment',
      'non-nullable',
      emptyProgramUri.toFilePath()
    ]);
  
  