Specs: make parentNode abstract in EventTarget

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
 import 'dart:collection';
 import 'dart:async';
 
 class ExceptionAndStackTrace<T> {
   const ExceptionAndStackTrace(this.exception, this.stackTrace);
   final T exception;
@@ skipping 127 matching lines @@
   // TODO(ianh): do events get retargetted, e.g. focus when leaving a custom element?
   // e.g. sent from inside a shadow tree, when exiting the shadow tree, focus event should:
   //  - disappear if we're moving from one to another element
   //  - be targetted if it's going to another node in a different scope
 }
 
 class EventTarget {
   EventTarget() : _eventsController = new DispatcherController<Event>();
 
   Dispatcher get events => _eventsController.dispatcher;
-  EventTarget parentNode;
+  EventTarget get parentNode;
 
   List<EventTarget> getEventDispatchChain() {
     if (this.parentNode == null) {
       return [this];
     } else {
       var result = this.parentNode.getEventDispatchChain();
       result.insert(0, this);
       return result;
     }
   }
@@ skipping 24 matching lines @@
       throw exceptions;
     return event.result;
   }
 
   void _dispatchEventLocally(Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
 ```