Add support for import@async

sky/engine/core/html/HTMLImportElement.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sky/engine/config.h"
 #include "sky/engine/core/html/HTMLImportElement.h"
 
 #include "gen/sky/core/EventTypeNames.h"
 #include "sky/engine/core/dom/Document.h"
 #include "sky/engine/core/events/Event.h"
@@ skipping 56 matching lines @@
 }
 
 void HTMLImportElement::importChildWasDestroyed(HTMLImportChild* child)
 {
     ASSERT(m_child == child);
     m_child = nullptr;
 }
 
 bool HTMLImportElement::isSync() const
 {
-    return true;
+    return !hasAttribute(HTMLNames::asyncAttr);
 }
 
 Element* HTMLImportElement::link()
 {
     return this;
 }
 
 }