[Flutter is] an open-source software development kit (SDK), developed by Google, to quickly build iOS and Android apps, sharing most of the code. It works in conjunction with the Android and iOS SDKs
Looks like a good alternative to React Native or Xamarin.
This article describes some of the advantages and drawbacks of Flutter:
To summarise: there’s lots of potential, it’s very easy to get started and actually create something real, and there are many good principles and ideas. However the community is still small and bits and pieces are missing in terms of cross-platform plugins, or there is not much choice in the best case. Also, you must be OK with the fact that you won’t have a 100 per cent native-looking UI, and that if you want to at least be as close as possible for both iOS and Android, your code and structure will get more complex.