[bug report] tree causes setcurrentkey (null) invalid problem in production environment

### Element UI version
2.6.3

### OS/Browsers version
win10/edge或者chrome

### Vue version
2.6.10

### Reproduction Link
https://elementui.github.io/issue-generator/#/zh-CN

### Steps to reproduce
在生产环境下, 点击树中不同节点, 设置高亮之后, 调用setCurrentKey(null)进行当前选中移除,移除后重新点击清除前的同一级或者下一级节点, 清除前的节点会被重新加上'is-current' 类名,而在开发环境不会;

### What is Expected?
setCurrentKey(null)清除当前选中之后 再次点击同一级节点或者下级节点,原先的选中节点不再添加'is-current' 类名

### What is actually happening?
setCurrentKey(null)清除当前选中之后 再次点击同一级节点或者下级节点,原先的选中节点重复添加'is-current' 类名