anchor.diff:      : _knownFlags = <String>[],
         _parser = new ArgParser();
   
  +
     /**
      * Defines a flag.
      *
  
positive.code:       return args;
      }
  
      // Filter all unrecognized flags and options.
      return args.where((arg) => !arg.startsWith('--') ||
          _knownFlags.contains(arg.substring(2)));
  
    }
  
  }
  
negative.code:         allowMultiple: allowMultiple);
    }
  
  
    /**
     * Generates a string displaying usage information for the defined options.
     *
  