WIP [esnext] implement async iteration proposal

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 758a726eb403daf288b042c8e8d6f803a76d36ed..ed75c396f0227d0ee028df9e7a7cb6d2f1d28e05 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -533,6 +533,8 @@ class V8_EXPORT_PRIVATE Factory final {
                                    size_t byte_offset, size_t byte_length);
 
   Handle<JSIteratorResult> NewJSIteratorResult(Handle<Object> value, bool done);
+  Handle<JSAsyncFromSyncIterator> NewJSAsyncFromSyncIterator(
+      Handle<HeapObject> sync_iterator);
 
   Handle<JSMap> NewJSMap();
   Handle<JSSet> NewJSSet();