Unable to update Dart SDK. Retrying…

C:\Users\linewell.sk>flutter doctor
Checking Dart SDK version...
Downloading Dart SDK from Flutter engine 3757390fa4b00d2d261bfdf5182d2e87c9113ff9...
Downloading the Dart SDK using the BITS service failed, retrying with WebRequest...
Invoke-WebRequest : 无效的 URI: 未能分析主机名。
所在位置 C:\Users\linewell.sk\flutter\bin\internal\update_dart_sdk.ps1:62 字符: 5
+     Invoke-WebRequest -Uri $dartSdkUrl -OutFile $dartSdkZip
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Invoke-WebRequest], UriFormatException
    + FullyQualifiedErrorId : System.UriFormatException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand
Error: Unable to update Dart SDK. Retrying...
等待 0 秒，按 CTRL+C 退出 ...

解决方案：
出现以上问题时，请查看自己的各个信息环境变相配置是否齐全，用户环境变量和系统环境变量都要检查一下。