Specs: Make appendChild, prependChild, and setChild return the child; make Dispatcher's unlisten() …

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 import 'dart:collection';
 import 'dart:async';
@@ skipping 37 matching lines @@
     if (_listeners == null)
       _listeners = new List<Pair<Handler, ZoneUnaryCallback>>();
     _listeners.add(new Pair<Handler, ZoneUnaryCallback>(handler, Zone.current.bindUnaryCallback(handler)));
   }
   bool unlisten(Handler<T> handler) {
     if (_listeners == null)
       return false;
     var target = _listeners.lastWhere((v) => v.a == handler, orElse: () => null);
     if (target == null)
       return false;
-    _listeners.remove(target);
+    _listeners.removeAt(_listeners.lastIndexOf(target));
     return true;
   }
   void _add(T data) {
     if (_listeners == null)
       return;
     ExceptionListException exceptions = new ExceptionListException();
     // we make a copy of the list here so that the listeners can
     // mutate our list without worry
     _listeners.toList().forEach((Pair<Handler, ZoneUnaryCallback> item) {
       try {
@@ skipping 110 matching lines @@
     return event.result;
   }
 
   void _dispatchEventLocally(Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
 </script>
 ```