Aren't those, with the exception of Elm, much younger and less mature than Dart? I think a language has to reach a certain level before the majority of people will use it in production. And the tooling should be stable also. I think PureScript looks great but it has a different syntax (Haskell like) which may make it a hard sell and I'm not sure they have any debugger at all. When you use a compile-to-js language I think the debugger and editors are important, at least if your language is not very close to js (which Dart is).