Flutter环境变量配置好之后，运行flutter doctor出现问题如下：

错误信息为：✗ HTTP host https://maven.google.com/ is not reachable. Reason: An error occurred while checking the HTTP host: Operation timed out

解决办法如下：

1、找到flutter sdk的文件目录，打开flutter\packages\flutter_tools\lib\src\http_host_validator.dart
2、将https://maven.google.com/ 修改为https://dl.google.com/dl/android/maven2/
3、去到flutter\bin目录，删除cache文件夹
4、重新打开终端窗口，重新执行flutter doctor。