Traversers doesn't handle the case of scala.scalajs.ir.Trees$Debugger

[neocxi]

scala.js is an awesome project! I tip my hat to the team. I encountered possibly an optimizer bug when I was trying out scala.js:

The issue seems to be that Traversers doesn't handle the case of scala.scalajs.ir.Trees$Debugger
https://github.com/scala-js/scala-js/blob/master/ir/src/main/scala/org/scalajs/core/ir/Traversers.scala#L193

Way to reproduce:
sbt run this repo https://github.com/neocxi/scalajs-angular-todomvc

Stacktrace:

Caused by: scala.scalajs.tools.optimizer.OptimizerCore$OptimizeException: The Scala.js optimizer crashed while optimizing Ltodomvc_example_TodoCtrl$.add__V
Methods attempted to inline:
* Ltodomvc_example_TodoCtrl$.scope__Lcom_greencatsoft_angularjs_core_Scope
* Ltodomvc_example_Task$.apply$default$2__Z
* Ltodomvc_example_Task$.apply$default$3__J
* sjsr_RuntimeLong$.apply__I__I__I__sjsr_RuntimeLong

    at scala.scalajs.tools.optimizer.OptimizerCore.optimize(OptimizerCore.scala:99) ~[na:na]
    at scala.scalajs.tools.optimizer.GenIncOptimizer$MethodImpl.process(GenIncOptimizer.scala:797) ~[na:na]
    at scala.scalajs.tools.optimizer.ParIncOptimizer$$anonfun$processAllTaggedMethods$2.apply(ParIncOptimizer.scala:82) ~[na:na]
    at scala.scalajs.tools.optimizer.ParIncOptimizer$$anonfun$processAllTaggedMethods$2.apply(ParIncOptimizer.scala:81) ~[na:na]
    at scala.collection.parallel.mutable.ParArray$ParArrayIterator.foreach_quick(ParArray.scala:145) ~[scala-library-2.11.2.jar:na]
Caused by: java.lang.RuntimeException: Invalid tree in traverse() of class class scala.scalajs.ir.Trees$Debugger
    at scala.sys.package$.error(package.scala:27) ~[scala-library-2.11.2.jar:na]
    at scala.scalajs.ir.Traversers$Traverser.traverse(Traversers.scala:206) ~[na:na]
    at scala.scalajs.tools.optimizer.OptimizerCore$RecreateInfoTraverser.traverse(OptimizerCore.scala:2734) ~[na:na]
    at scala.scalajs.ir.Traversers$Traverser$$anonfun$traverse$1.apply(Traversers.scala:26) ~[na:na]
    at scala.scalajs.ir.Traversers$Traverser$$anonfun$traverse$1.apply(Traversers.scala:26) ~[na:na]

[sjrd]

Indeed, looks like we overlooked that one.