原因是我直接从External Libraries 上，将安装包删掉了，然后在pubspec.yaml 中再次添加这个包，都安装不了了，运行了 Flutter Pub get
和 Flutter Pub upgrade ，Flutter Pub clean ，包括重启编译器，都不行，最后试了如下命令，就可以安装成功了，做个记录：
flutter pub cache repair
I have a similar problem I deleted a package from the dart packages folder and when I run flutter pub run it won’t download and work so I tried all the way people said but something worked for me is I tried to repairing pub cache and it worked just run.