On the material icons there are previews of each icon:

https://api.flutter.dev/flutter/material/Icons-class.html

However the same is not the case for the Cupertino icons:

https://api.flutter.dev/flutter/cupertino/CupertinoIcons-class.html

I don't know if there's a technical reason for this - it'd be nice to support if possible. In VS Code we use a regex to find these icon previews for Material and swap them out with images that will work there (so you have in-editor previews), but this also fails for Cupertino. If there were previous on the page, we could probably do the same thing there (raised at Dart-Code/Dart-Code#638 (comment)).