Specs: dartification of dom.md, more work on script.md; this is highly work-in-progress and hasn't …

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
 SKY MODULE
 <!-- part of sky:core -->
 
 <script>
 abstract class Event<ReturnType> {
   Event() { init(); }
@@ skipping 11 matching lines @@
   ReturnType result;
 
   bool resultIsCompatible(dynamic candidate) => candidate is ReturnType;
 
   // TODO(ianh): abstract API for doing things at shadow tree boundaries 
   // TODO(ianh): do events get blocked at scope boundaries, e.g. focus events when both sides are in the scope?
   // TODO(ianh): do events get retargetted, e.g. focus when leaving a custom element?
 }
 
 class EventTarget {
-  EventTarget() : _eventsController = new DispatcherController<Event>();
+  EventTarget() : _eventsController = new DispatcherController<@nonnull Event>();
 
   Dispatcher get events => _eventsController.dispatcher;
   EventTarget parentNode;
 
-  List<EventTarget> getEventDispatchChain() {
+  List<@nonnull EventTarget> getEventDispatchChain() {
     if (this.parentNode == null) {
       return [this];
     } else {
       var result = this.parentNode.getEventDispatchChain();
       result.insert(0, this);
       return result;
     }
   }
 
   final DispatcherController _eventsController;
 
-  dynamic dispatchEvent(Event event, { defaultResult: null }) { // O(N*M) where N is the length of the chain and M is the average number of listeners per link in the chain
+  dynamic dispatchEvent(@nonnull Event event, { dynamic defaultResult: null }) { // O(N*M) where N is the length of the chain and M is the average number of listeners per link in the chain
     // note: this will throw an ExceptionListException<ExceptionListException> if any of the listeners threw
     assert(event != null); // event must be non-null
     event.handled = false;
     assert(event.resultIsCompatible(defaultResult));
     event.result = defaultResult;
     event._target = this;
     var chain;
     if (event.bubbles)
       chain = this.getEventDispatchChain();
     else
       chain = [this];
     var exceptions = new ExceptionListException<ExceptionListException>();
     for (var link in chain) {
       try {
         link._dispatchEventLocally(event);
       } on ExceptionListException catch (e) {
         exceptions.add(e);
       }
     }
     if (exceptions.length > 0)
       throw exceptions;
     return event.result;
   }
 
-  void _dispatchEventLocally(Event event) {
+  void _dispatchEventLocally(@nonnull Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
 </script>
 ```