I'm not going to argue against your point though, because I think you are right. One needs to have some grasp of the JS ecosystem to know what libraries to use. And as I said, it's nice that Dart has made that choice for you.
But that said, poking around a bit in the Dart documentation I think it's interesting that "Futures" are seemingly /not/ entirely interoperable with the de facto standard of Promises (A+) in JS.