The first step would probably be development of a cbindgen equivalent for Dart. I.e. generate the Dart
ffi binding code from the Rust declarations (or, I suppose, C header). I don't think such a tool exists.
I think C header -> Dart declaration would be best, so we'd go Rust -> C -> Dart. After all, it is the C ABI we're communicating over. Such a tool should probably be written in communication with the Dart community and, depending on relative support for the workload, potentially written in Dart. (After all, the Dart community would know more about the minutia of the task, and are more likely to be able to contribute to a Dart project than a Rust one.)
After that, it's a matter of providing ways of doing the actual interop. Theoretically, it's as simple as making sure the declarations on both sides match, then just compiling and linking both ends.
I'm (very slowly) learning Dart/Flutter in some of my spare time (though I have plenty of JVM experience with Java/Kotlin), and would love to help out with such a project. Someone with at least some experience in the Dart community (such as where you'd actually advertise the dart-bindgen tool) should probably lead the project, though.