Flutter:Failed to retrieve the Dart SDK from:

Downloading Dart SDK from Flutter engine c4229bfbbae455ad69c967be19aee3fadd6486e1...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:03:36 --:--:--     0
curl: (35) LibreSSL SSL_connect: SSL_ERROR_SYSCALL in connection to storage.googleapis.com:443 

Failed to retrieve the Dart SDK from: https://storage.googleapis.com/flutter_infra/flutter/c4229bfbbae455ad69c967be19aee3fadd6486e1/dart-sdk-darwin-x64.zip
If you're located in China, please see this page:
  https://flutter.dev/community/china

执行flutter doctor之前，先执行一下命令
export PUB_HOSTED_URL=https://pub.flutter-io.cn
export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn