Events should have an initialized flag.

third_party/WebKit/Source/core/events/EventTarget.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  *           (C) 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 238 matching lines @@
 bool EventTarget::clearAttributeEventListener(const AtomicString& eventType)
 {
     EventListener* listener = getAttributeEventListener(eventType);
     if (!listener)
         return false;
     return removeEventListener(eventType, listener, false);
 }
 
 bool EventTarget::dispatchEventForBindings(Event* event, ExceptionState& exceptionState)
 {
-    if (event->type().isEmpty()) {
+    if (!event->wasInitialized()) {
         exceptionState.throwDOMException(InvalidStateError, "The event provided is uninitialized.");
         return false;
     }
     if (event->isBeingDispatched()) {
         exceptionState.throwDOMException(InvalidStateError, "The event is already being dispatched.");
         return false;
     }
 
     if (!getExecutionContext())
         return false;
@@ skipping 83 matching lines @@
                 UseCounter::count(executingWindow->document(), prefixedFeature);
         } else if (listenersVector) {
             UseCounter::count(executingWindow->document(), unprefixedFeature);
         }
     }
 }
 
 DispatchEventResult EventTarget::fireEventListeners(Event* event)
 {
     ASSERT(!EventDispatchForbiddenScope::isEventDispatchForbidden());
-    ASSERT(event && !event->type().isEmpty());
+    DCHECK(event);
+    DCHECK(event->wasInitialized());
 
     EventTargetData* d = eventTargetData();
     if (!d)
         return DispatchEventResult::NotCanceled;
 
     EventListenerVector* legacyListenersVector = nullptr;
     AtomicString legacyTypeName = legacyType(event);
     if (!legacyTypeName.isEmpty())
         legacyListenersVector = d->eventListenerMap.find(legacyTypeName);
 
@@ skipping 116 matching lines @@
     // they have one less listener to invoke.
     if (d->firingEventIterators) {
         for (size_t i = 0; i < d->firingEventIterators->size(); ++i) {
             d->firingEventIterators->at(i).iterator = 0;
             d->firingEventIterators->at(i).end = 0;
         }
     }
 }
 
 } // namespace blink