Looks like Dart is getting a lot of popularity nowadays. In fact, features of Dart is the sum total of Java and Javascript/Typescript features minus all unwanted baggage those languages carry. Also, the package ecosystem around it is quite agile.
I have a feeling that future browsers may provide built-in support for Dart (right now also, Dart to Javascript conversion is part of Dart tools).
Vaadin recently started supporting Typescript and on the same lines, direct support for Dart should be possible in the future for the front-end. This way, it can even support Flutter because flutter is just Dart only.
The server part of Vaadin is pure Java and because of that it can support any JVM based languages. However, Dart is not JVM based.
Just some food for thought.