从git 获取flutter sdk

git clone -b stable https://github.com/flutter/flutter.git

如果报以下错误：
error: RPC failed; curl 7 LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 60
fatal: expected flush after ref listing

多方面查找，发现执行以下命令可行：

第一步：pod repo remove master
第二步：git clone https://git.coding.net/CocoaPods/Specs.git ~/.cocoapods/repos/master