Specs: define how importing element registrations works, move built-in elements to a separate modul…

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
-SKY MODULE
-<!-- part of dart:sky -->
-
-<script>
 import 'dart:collection';
 import 'dart:async';
 
 class ExceptionAndStackTrace<T> {
   const ExceptionAndStackTrace(this.exception, this.stackTrace);
   final T exception;
   final StackTrace stackTrace;
 }
 
 class ExceptionListException<T> extends IterableMixin<ExceptionAndStackTrace<T>> implements Exception {
@@ skipping 173 matching lines @@
     if (exceptions.length > 0)
       throw exceptions;
     return event.result;
   }
 
   void _dispatchEventLocally(Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
-</script>
 ```