Flutter SDK变更后的处理方式

Flutter SDK变更后处理不好，执行时就会提示下面的警告：

Warning! This package referenced a Flutter repository via the .packages file that is no longer available. The repository from which the 'flutter' tool is currently executing will be used instead.

running Flutter tool: /Users/woodstream/Documents/flutter/flutter
previous reference : /Users/woodstream/Documents/xxxx/flutter

网上有些方法用了未必有效。比如有说直接修改项目中的.packages文件，改完后可能还会有问题。

其实很简单，执行两步即可：

1.修改环境变量。

2.执行flutter package upgrade。