Bug 1420961 part 2 - Replace if-statement with an assert in js::GetIterator. r=evilpie

js/src/jsiter.cpp

@@ -876,8 +876,7 @@ js::GetIterator(JSContext* cx, HandleObject obj, unsigned flags)
             numGuards = 0;
     }
 
-    if (MOZ_UNLIKELY(obj->is<PropertyIteratorObject>()))
-        return obj;
+    MOZ_ASSERT(!obj->is<PropertyIteratorObject>());
 
     // We should only call the enumerate trap for "for-in".
     // Or when we call GetIterator from the Proxy [[Enumerate]] hook.