Kotlin and Dart both compile to JS. But on the server, Kotlin's Java integration means that you can leverage the maturity and large amount of existing libraries. Dart has pretty much nothing right now and will take a very long time to build up that level of support, if it ever does. Kotlin on the other hand is relatively risk free in the sense that there's going to be VMs on server platforms for any reasonable timeframe. Even if Kotlin is abandoned, you wouldn't have to kill you code base, and you could slowly move code over to Java or Scala or whatever other language you want.
“nobody needs Objective-C interop except people who build apps for Apple”
This is a fairly silly statement. I could just as easily say “nobody needs JS interop except for people whoe build web apps”. iOS is a massive customer base, and big enough that it’s worth trying to improve the state of app development. Whether it’s the right thing to be spending time on is of course up for debate (I’m more of a server developer, so I’d prefer to see the VM progress), but using Kotlin for iOS app development would be pretty awesome.
“But you must have a guess on when you think it can be used on real applications.”
While not in production, I’ve got a pretty involved web application built with Kotlin that I’ve been developing for a few months and I don’t see any issues that would prevent it from going into production. Yes, there have been some bumps in the road, but those issues come up most at compile time. It usually just means using slightly different code. And even that hasn’t happened too much lately. There’s still stuff that’s not that that I’d love to have sooner rather than later (modules), but nothing has really gotten in my way.