Dart/Flutter are very interesting but I think that there might already be a replacement: SWIFT.
Swift is being used by Apple of course, but there might be plans for an Android development :
Infoworld news
Even if you are dubious of this happening, there's a company (Elements) that offers a
free tool-chain to develop (on major platforms) in their porting of SWIFT and generate your targeted platform app (but it's
not cross-platform)
I have started playing with it (using their Visual Studio plugin, but they offer a free IDE for Macs) and it's actually pretty good. The UI elements are not shareable but that might be an advantage. You can import/convert your current java, Objective-C and C# into Swift.
If Google were to drop java for Android development, I have no doubt that this company would adapt their tool to target the new language.
Here's more info on their tool:
Silver if you are interested.
The Frameworks
Silver embraces each platform's native SDKs and Frameworks. So you're leveraging the native Java and Dalvik APIs on Android, working directly against the .NET and WinRT frameworks on Microsoft's platform, and creating true and native Cocoa apps for Mac, iOS, watchOS and tvOS. All using the Swift language.
At the same time, our (optional)
Sugar cross-platform library allows you to share large amounts of back-end code and keep it cross-platform – especially helpful if you are creating "the same" app for different devices.
Sugar is open source, so you can even contribute and extend the library.