good thing to have in an FFI system is ability from one language to call another from callback, and back and forth (if it need to). I believe Dart allows for that. That to be said, stack walking is going to be painful to decode (probably), best I've seen is C#/C++ on Windows - where it flows naturally from one to another.
dart:ffi (https://dart.dev/guides/libraries/c-interop) is a recent addition to Dart and from where I stand it's a decent FFI system.
Maybe not as good as PInvoke in C# or ffi in luajit but better than, say, jni in Java (and any other FFI system that requires you to write glue code in C i.e. python, ruby, nodejs).