WIP [esnext] implement async iteration proposal

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index c79edcd27e68c1b353faa0d646f36f9626be27e7..07d3ac5bc6cc440f89ec98913b74c78e0ecb205d 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -538,6 +538,8 @@ 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();