docs: function mapItems(dict, fn, opt_this) {
      opt_this = opt_this || this;
      var result = {};
      var keys = Object.keys(dict);
      for (var i = 0; i < keys.length; i++) {
        var key = keys[i];
        result[key] = fn.call(opt_this, key, dict[key]);
      }
      return result;
    }