Flutter 开发环境搭建Unable to ‘pub upgrade‘ flutter tool. Retrying in five seconds...

好久没更新Flutter专栏了，一是因为懒。而是因为在公司的Windows 10上配置flutter环境总是出错，然后就以此为借口懒得弄了，这不Flutter 2.0出现了，又燃起了我学习Flutter的热情。今天周末来公司加班，就一举搞定了flutter的开发环境配置问题。

配置流程如下：
1、在github上搜flutter，然后将其clone到本地
2、配置好环境变量
3、配置好镜像，配置好以后需要重启电脑，这一点很重要，如果不配置镜像的话，会出现“Unable to ‘pub upgrade’ flutter tool. Retrying in five seconds…”的错误。

E:\flutter\bin>flutter doctor
Checking Dart SDK version...
Downloading Dart SDK from Flutter engine ...
Downloading the Dart SDK using the BITS service failed, retrying with WebRequest...
Building flutter tool...
Running pub upgrade...
Got TLS