code: function () {
              if (this._multiple) {
                  var /** @type {?} */ selectedOptions = this._selectionModel.selected.map(function (option) { return option.viewValue; });
                  if (this._isRtl()) {
                      selectedOptions.reverse();
                  }
                  // TODO(crisbeto): delimiter should be configurable for proper localization.
                  return selectedOptions.join(', ');
              }
              return this._selectionModel.selected[0].viewValue;
          }
docs: The value displayed in the trigger.
  @return {?}