Optimize js.Array ops in Scala.js

[Scala.js Fiddle](http://www.scala-js-fiddle.com/) includes Scalaxy/Loops by default!!!

[js.Array](http://www.scala-js.org/api/scalajs-library/0.6.2/#scala.scalajs.js.Array) seems like an ideal target for optimizations: adding a source and a sink should just be enough.
