First Make sure you have a working internet connection and if this issue still exists then, this error may be because the dart sdk may have not been properly downloaded. The simplest solution is to delete the `cache` folder located in `flutter/bin`(location where flutter is installed) and then Run `flutter doctor`

First Make sure you have a working internet connection and if this issue still exists then, this error may be because the dart sdk may have not been properly downloaded. The simplest solution is to delete the ```cache``` folder located in ```flutter/bin```(location where flutter is installed) and then Run ```flutter doctor```