我开始使用 Flutter 教程，但是在运行 get package 时它会让我跟踪错误
> C:\Users\pacif\Desktop\flutter\bin\flutter.bat --no-color packages upgrade
> Running "flutter packages upgrade" in flutter_app...
> Error on line 26, column 8 of pubspec.yaml: Invalid version constraint: > Could not parse version "flutter". Unknown text at "flutter".
> sdk: flutter
> ^^^^^^^
pub 升级失败(65)进程以退出代码 65 结束