Update index.js

[cy-biggray]

现在生成的虚拟列表没有key值，每次滑动时，vue都需要更新。

What kind of this PR? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Build-related changes
• [ ] Other, please describe:

Other information:

[cy-biggray]

由于没有key值，每次滑动时，oldChildList和childList都需要重新patch。特别影响性能