Google has released Flutter 1.0, its cross-platform app UI toolkit for developers.
Flutter is not a replacement for the operating system-specific app models, but a way developers can use the app engine to embed in an existing app, or use for a new app.

Google claims it's fast, and says it's an open source project.

[developers.googleblog.com...]

Earlier on this topic
Google's New Flutter Mobile UI Framework SDK is Now Released in Beta [webmasterworld.com]