执行flutter doctor报错
[!] HTTP Host Availability
✗ HTTP host https://maven.google.com/ is not reachable. Reason: An error
occurred while checking the HTTP host: Operation timed out
解决方法：
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/，保存退出
3.在flutter\bin目录，删除cache文件夹
4.重新运行终端执行flutter doctor，自动下载文件，解决问题
