Thanks to a collaboration between Google and Adobe, it's now possible to use the Adobe XD design tool to create UI for Flutter apps.

03-18-2016 DESIGN Adobe XD Is A Slick UX Design Tool With Zero Learning Curve Free to try, Adobe XD allows you to go from wireframe to interactive prototype, from desktop to mobile, in seconds.

The Adobe XD Starter Plan includes a desktop version on both Mac and Windows 10, full design and prototyping capabilities and mobile preview apps on iOS and Android.