anchor.diff:        if (option.help != null) write(2, option.help);
   
         if (option.allowedHelp != null) {
  -        var allowedNames = option.allowedHelp.getKeys();
  +        var allowedNames = option.allowedHelp.keys;
           allowedNames.sort((a, b) => a.compareTo(b));
           newline();
           for (var name in allowedNames) {
  
positive.code: 
        // Make room for the allowed help.
        if (option.allowedHelp != null) {
          for (var allowed in option.allowedHelp.getKeys()) {
            title = max(title, getAllowedTitle(allowed).length);
          }
        }
  
negative.code:   }
  
    /** Get the names of the options as a [Collection]. */
    Collection<String> get options => _options.keys;
  }
  
  class _Option {
  