download dart sdk from flutter engine code example
Example: Downloading Dart SDK from Flutter engine on checking flutter version
This error comes if you stopped the "flutter upgrade" command and the
installation is not completed.
Solution: if you have the last version zip then replacing it with current or
adding that to new folder and then update the env PATH will solve this issue.
Or download zip file from here https://flutter.dev/docs/get-started/install/windows
and follow the same process as above