code: function RegExpGetMultiline() {
    if (!IS_REGEXP(this)) {
      // TODO(littledan): Remove this RegExp compat workaround
      if (this === GlobalRegExpPrototype) {
        %IncrementUseCounter(kRegExpPrototypeOldFlagGetter);
        return UNDEFINED;
      }
      throw MakeTypeError(kRegExpNonRegExp, "RegExp.prototype.multiline");
    }
    return TO_BOOLEAN(REGEXP_MULTILINE(this));
  }
docs: ES6 21.2.5.7.