WIP [esnext] implement async iteration proposal

src/builtins/builtins-function.cc

Index: src/builtins/builtins-function.cc
diff --git a/src/builtins/builtins-function.cc b/src/builtins/builtins-function.cc
index 818e09a722590e84d12357d19109683a06370bbc..ab915b0a675e6e5b8b5f8c20e553b50ef6063906 100644
--- a/src/builtins/builtins-function.cc
+++ b/src/builtins/builtins-function.cc
@@ -163,6 +163,24 @@ BUILTIN(AsyncFunctionConstructor) {
   return *func;
 }
 
+BUILTIN(AsyncGeneratorFunctionConstructor) {
+  HandleScope scope(isolate);
+  Handle<Object> maybe_func;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, maybe_func,
+      CreateDynamicFunction(isolate, args, "async function*"));
+  if (!maybe_func->IsJSFunction()) return *maybe_func;
+
+  // Do not lazily compute eval position for AsyncFunction, as they may not be
+  // determined after the function is resumed.
+  Handle<JSFunction> func = Handle<JSFunction>::cast(maybe_func);
+  Handle<Script> script = handle(Script::cast(func->shared()->script()));
+  int position = script->GetEvalPosition();
+  USE(position);
+
+  return *func;
+}
+
 namespace {
 
 Object* DoFunctionBind(Isolate* isolate, BuiltinArguments args) {