Realtime message subscription

[tiholic]

Listening to messages on a Realtime Channel

This implementation follows dart idiomatic approach

Subscribing

Stream<ably.Message> messageStream = channel.subscribe();
StreamSubscription<ably.Message> channelMessageSubscription = messageStream.listen((ably.Message message){
  print("New message arrived ${message.data}");
});

Unsubscribing

await channelMessageSubscription.cancel();

[QuintinWillison]

I've been offline for a few hours this afternoon so apologies for the late review submit. I wrote most of the comments in the AM but only just submitting the review as a whole now.

[zoechi]

Some suggestions