You may heard about flutter somewhere before but if you don't know it, here is a summary.
Flutter is a cross-platform mobile framework written in Dart language, it doesn't use any platform bridge, it contacts directly with the platform itself! that is why it is faster than ReactNative, NativeScript ... etc.
You can write flutter platform specific modules in java/kotlin/swift/Objective-C using its PlatformChannel API.
Google using flutter to drive the entire UI for the upcoming upcoming Fuchsia OS, not only that, but also the community trying to release desktop embeds so you can write the code once and deploy it for mobile or desktop!
Regarding productivity, you can write a fully functional app in less than 500 line of code for both Android & IOS without any hassle!
In uFlare, our team now aware of flutter and currently we using it in production.