anchor.diff:  
     DocCommand({bool verbose = false}) : super(cmdName, cmdDescription, verbose) {
       argParser.addOption(
  -      'output-dir',
  +      'output',
         abbr: 'o',
  +      valueHelp: 'directory',
         defaultsTo: path.join('doc', 'api'),
  -      help: 'Output directory',
  +      aliases: [
  +        // The CLI option that shipped with Dart 2.16.
  +        'output-dir',
  +      ],
  +      help: 'Configure the output directory.',
       );
       argParser.addFlag(
         'validate-links',
  -      negatable: true,
  -      help: 'Display context aware warnings for broken links (slow)',
  +      negatable: false,
  +      help: 'Display warnings for broken links.',
       );
     }
   
  
positive.code:       return 2;
      }
  
      // Call dartdoc.
      if (verbose) {
        log.stdout('Using the following options: $options');
      }
  
      final packageConfigProvider = PhysicalPackageConfigProvider();
      final packageBuilder = PubPackageBuilder(
          config, pubPackageMetaProvider, packageConfigProvider);
  
negative.code:       usageException("Error: Input directory doesn't exist: ${dir.path}");
      }
  
      // Specify where dartdoc resources are located.
      final resourcesPath =
          path.absolute(sdk.sdkPath, 'bin', 'resources', 'dartdoc', 'resources');
  
      // Build options.
      final options = [
        '--input=${dir.path}',
        '--output=${argResults['output']}',
        '--resources-dir=$resourcesPath',
        if (argResults['validate-links']) '--validate-links'
      ];
  
      final config = await parseOptions(pubPackageMetaProvider, options);
      if (config == null) {
  