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：

1. 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/
2. 删除/path-to-flutter-sdk/bin/cache 文件夹
3. 重新执行flutter doctor

参考: https://zhuanlan.zhihu.com/p/480002331