I only tested the Android version and it is still extremely early.
Very basic samples that don't run very well and implement Material elements in an uncanny valley kind of way compared to the existing web & mobiles implementations.
Not to mention that Android with Java/Kotlin has a truckload of tooling that Dart would have to replace.
There are several parts of the android framework that I would like to see rewritten from scratch (activity as a god object, love of inheritance everywhere in the framework classes, weak UI/animation framework, etc) but it is also a mature code base with tons of features, tools & open source libraries built around it.