How do I pass multiple parameter from JavaScript to Dart through a Flutter WebView plugin JavaScript Channel?

Let’s say I have a channel named setPosition. I want it to accept 2 parameters x and y and do something with them. I think I should do setPosition.postMessage(x, y) in JavaScript, but then what do I do under onMessageReceived in Dart?

I have Googled this a few times and found nothing so far.