A modern programming language that makes developers happier.

kotlinlang

I personally think that it's impossible (or at least quite difficult) to get away with having no platform-specific experts and testing with cross-platform.  However, I do think it makes sense to share a lot of code _including layouts_ if you leave styling to the platform and make it _extremely easy_ to access the platform directly.  Kotlin has a huge advantage in this sense - the language has access to the platform's actual APIs with no layers inbetween.
I think the key is making a cross-platform view library, as opposed to a framework.  If you can access the layer underneath, you can take care of platform quirks and extend your tools while also keeping the convenience of defining the truly duplicated stuff across platforms.