retain canonical reverse order;

--- a/src/Pure/System/event_bus.scala	Thu Apr 24 10:30:55 2014 +0200
+++ b/src/Pure/System/event_bus.scala	Thu Apr 24 10:33:06 2014 +0200
@@ -28,5 +28,5 @@
 
   /* event invocation */
 
-  def event(x: Event) { receivers.value.reverseIterator.foreach(_ ! x) }
+  def event(x: Event) { receivers.value.iterator.foreach(_ ! x) }
 }