Check out example codes for "unable to update dart sdk. retrying". It will help you in understanding the concepts better.
Code Example 1
Add below path to Environment variable User Variables C:\flutter\bin; C:\Windows\System32 %SYSTEMROOT%\System32\WindowsPowerShell\v1.0\ System Variables C:\flutter\bin C:\Windows\System32\
Code Example 2
restart your pc and delete flutter folder in C directroy then reinistall flutter and run flutter doctor
Learn ReactJs, React Native from akashmittal.com