I think dart/flutter serves primarily two purposes, as I see it. First the obvious one, Google is trying to get devs used to flutter as when the time comes to switch to Fuchsia/Zircon, it would be relatively a breeze.
Secondly, and the most immediate reason, I think it's a very clever and clear move from Google to lock-in devs into using Google Cloud services like Firebase and Cloud ML services by leveraging Flutter. If you have observed, Microsoft has given a rebirth to their Xamarin developers YouTube channel and have been hosting 'The Xamarin Show', which is, hmm, quite similar to 'The Boring Flutter Development Show'.
Secondly, and the most immediate reason, I think it's a very clever and clear move from Google to lock-in devs into using Google Cloud services like Firebase and Cloud ML services by leveraging Flutter. If you have observed, Microsoft has given a rebirth to their Xamarin developers YouTube channel and have been hosting 'The Xamarin Show', which is, hmm, quite similar to 'The Boring Flutter Development Show'.