With the JVM of the near future [0] running JavaScript natively, that could change again. Long before you know it, you'd be running java-scala-clojure-javascript code inside of the same jvm on the server, and on the client...
It's already out in Java 8 but the parser can choke on large js files. I tried loading some Ceylon code compiled to js into the webview but it froze. I think it will have the same problem with Scala.js