flutter pub get缓慢解决方案 windows 
set PUB_HOSTED_URL=https://pub.flutter-io.cn
set FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn

flutter pub get
Linux MACOS 
export PUB_HOSTED_URL=https://pub.flutter-io.cn
export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn

flutter pub get