Since ffigen added support for
[`NativeCallable.listener`](https://api.flutter.dev/flutter/dart-ffi/NativeCallable/NativeCallable.listener.html)
to its ObjC bindings, this example can be simplified. We can replace the
`Dart_Port` logic with `ObjCBlock.listener`, which lets us get rid of
most of the native code.
We still need a small bit of native code to `retain` a reference to the
callback's arguments before invoking the listener, otherwise the
arguments may be ref counted and deleted before the Dart side of the
callback is invoked. See https://github.com/dart-lang/native/issues/835
