Recapping Flutter 1.0 announcements: blog 
The primary target for Flutter has so far been iOS and Android. Yet our ambitions for Flutter extend beyond mobile to a broader set of platforms. Indeed, from the outset Flutter was architected as a portable UI toolkit that is flexible enough to go wherever pixels are painted.
Hummingbird is an experimental web-based implementation of the Flutter runtime that takes advantage of the capability of the Dart platform to compile not just to native ARM code but also to JavaScript. This enables Flutter code to run on the standards-based web without change.
Observing Dart and transition to Capabilities in web and other UI development. Google is being very smart building the developer ecosystem for mobile before fitting it on to a new capabilities based microkernel. An exercise in persuading developer adoption through extreme agile.
See Flutter Client for FedWiki.