Flutter requires you to write your code using the flutter language (which is a completely different kind of programming from java as it is declarative instead of imperative). For anything native that isn't supported by flutter, you have to write the native code and then add the glue to tie it all together. I utilize a large amount of OS specific functionality in each version, so I'd be running up against this problem repeatedly. The last time I checked, flutter also doesn't support all of the native stuff I would need. I do appreciate you bringing it up though, as more and more cross-platform frameworks are being introduced. A lot of people like react native, but once again, it has limitations and drawbacks depending upon what you want to do, and you run into problems when utilizing more complex platform specific functionality.
Mike
Mike