Flutter 更新失败问题

今天在执行flutter upgrade更新flutter时发现了一个更新失败问题：

1	Failed to retrieve the Dart SDK from: https://storage.googleapis.com/flutter_infra/flutter/6bc433c6b6b5b98dcf4cc11aff31cdee90849f32/dart-sdk-darwin-x64.zip

发生这个问题的主要原因就是这个地址被墙了，官方也给出了解决方案，就是给更新地址设置 mirror site，具体的方法就是在 Bash shell 中做如下设置：

export PUB_HOSTED_URL=https://pub.flutter-io.cn
export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn
git clone -b dev https://github.com/flutter/flutter.git
export PATH="$PWD/flutter/bin:$PATH"
cd ./flutter
flutter doctor

除了这个 mirror site 之外，还可以用 Shanghai Jiaotong University Linux User Group 的 mirror site：

1 2	FLUTTER_STORAGE_BASE_URL: https://mirrors.sjtug.sjtu.edu.cn/ PUB_HOSTED_URL: https://dart-pub.mirrors.sjtug.sjtu.edu.cn/

设置好之后就可以发会呆等待其 download 完成了。