I'm not worried about learning Dart. I'm worried about adding yet another tooling and OSS ecosystem to my collection, that's where the real cognitive burden is. I'll do it if I have to, but in mobile you really don't have to. You can choose from Java/Kotlin, Swift, .NET, and HTML+(Java/Type)Script - chances are you're already in one of these worlds, and if you're not then entering any one of them will pay much bigger dividends than Dart.