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..4feffb79ace6065c14f37e907c5b3d13baed4282 100644
--- a/src/builtins/builtins-promise.h
+++ b/src/builtins/builtins-promise.h
@@ -26,6 +26,11 @@ class PromiseBuiltinsAssembler : public CodeStubAssembler {
   // init hook.
   Node* AllocateAndInitJSPromise(Node* context, Node* parent);
 
+  // Allocate and initialize a Promise with pending state and undefined fields,
+  // without invoking Promise Hooks. This is suitable for internal Promises
+  // not exposed to JS code.
+  Node* AllocateAndInitInternalJSPromise(Node* context);
+
   // This allocates and initializes a promise with the given state and
   // fields.
   Node* AllocateAndSetJSPromise(Node* context, Node* status, Node* result);
@@ -57,6 +62,11 @@ 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,