Well. Flutter is just a Framework (just like Django btw.) and it uses Dart as a programming language. Django is kind of solving a different problem than Dart. Dart is intened for use in Front End Applications and Django is a Framework for Back-End Web Development.
So if you want to program Flutter Apps (although i wouldn't recommend it for any serious web development yet since Flutter web isn't very mature yet) i would recommend you just lern Dart.