最近在学习flutter，有时从GitHub下载开源项目学习，运行项目时会遇到flutter SDK版本太低的情况，这时就需要升级SDK。
The current Dart SDK version is 2.1.0-dev.0.0.flutter-be6309690f.
Because buddy depends on flutter_built_redux 0.4.5 which requires SDK
version >=2.2.0 <3.0.0, version solving failed.
pub get failed (1)
升级 Flutter channel 和 packages
要同时更新Flutter SDK和你的依赖包，在你的应用程序根目录（包含pubspec.yaml文件的目录）中运行flutter upgrade 命令:$ flutter upgrade
实际运行时会执行失败，结果如下图：
Pasted Graphic 5.png
提示你中国地区用户参考：https://flutter.dev/community/china
$ 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
执行成功结果如下：
Pasted Graphic 4.png
