This is damage control, save yourself trouble now and jump ship away from Flutter/Dart. There are other options for cross-platform: Xamarin, React Native, and Delphi (I use Delphi, there is a community version and a payed version... they are both excellent and I am able to target Desktop and Mobile easily). If you don't want either luckily Kotlin and Swift are very similar and you will just need to build two applications.