WIP [esnext] implement async iteration proposal

src/builtins/builtins-promise.h

Index: src/builtins/builtins-promise.h
diff --git a/src/builtins/builtins-promise.h b/src/builtins/builtins-promise.h
index 00d57c73b1d68e90a2000bbc94647523a031de37..ff5496775f760e4f00d623a38f700b702cf8078c 100644
--- a/src/builtins/builtins-promise.h
+++ b/src/builtins/builtins-promise.h
@@ -57,6 +57,11 @@ class PromiseBuiltinsAssembler : public CodeStubAssembler {
                                    Node* deferred_on_resolve,
                                    Node* deferred_on_reject);
 
+  void FastPerformPromiseThen(Node* context, Node* promise,
+                              Node* on_resolve = nullptr,
+                              Node* on_reject = nullptr,
+                              Node* result_promise = nullptr);
+
   void InternalResolvePromise(Node* context, Node* promise, Node* result);
 
   void BranchIfFastPath(Node* context, Node* promise, Label* if_isunmodified,