android 5.1 报错 $imports is not defined
webpack-dev-server 开发时报错

![image](https://user-images.githubusercontent.com/23187848/59506663-67162f80-8edb-11e9-9499-9a1915ec3f12.png)
![image](https://user-images.githubusercontent.com/23187848/59508120-726b5a00-8edf-11e9-89c4-255ef12e34aa.png)

