I agree that Kotlin is the superior language, it truly is wonderful to work with. That said, Dart does have type safety and also null safety (coming in now).
I can see Kotlin Multiplatform working for bigger teams when you really want that native look and feel and you have significant business logic that can be shared between platforms. Such teams will have continuous integration pipelines and test suites.
For the many, many small development shops though the ergonomics would not work. The write / deploy / test cycle here must be lightning fast.