Merge pull request #22 from sjrd/final-val-undefined

Make `Undefined` a `final val` so that it can be constant-folded.

Author: djspiewak

core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala

@@ -26,7 +26,7 @@ import scala.util.control.NonFatal
  * Based on https://github.com/YuzuJS/setImmediate
  */
 object MacrotaskExecutor extends ExecutionContextExecutor {
-  private[this] val Undefined = "undefined"
+  private[this] final val Undefined = "undefined"
 
   def execute(runnable: Runnable): Unit =
     setImmediate(() => runnable.run())