Add support for import@async

We don't have a good way of testing this behavior without introducing a race.
Ideally we'd unit test it, but we don't really have unit tests yet.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/751493003

engine/core/html/HTMLImportElement.cpp

@@ -74,7 +74,7 @@ void HTMLImportElement::importChildWasDestroyed(HTMLImportChild* child)
 
 bool HTMLImportElement::isSync() const
 {
-    return true;
+    return !hasAttribute(HTMLNames::asyncAttr);
 }
 
 Element* HTMLImportElement::link()