HTTP Host Availability
错误提示如下，分析可知是 maven.google.com 访问失败造成的：
HTTP host https://maven.google.com/ is not reachable. Reason: An error occurred while checking the HTTP host: Operation timed out
解决步骤：
- 打开
/path-to-flutter-sdk/packages/flutter_tools/lib/src/http_host_validator.dart文件，修改https://maven.google.com/为 google maven 的国内镜像，如https://maven.aliyun.com/repository/google/ - 删除
/path-to-flutter-sdk/bin/cache文件夹 - 重新执行
flutter doctor
