The current Dart SDK version is 2.10.4. flutter 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