[esnext] prototype runtime implementation for async functions

src/builtins.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins.h"
 
 #include "src/api-arguments.h"
 #include "src/api-natives.h"
 #include "src/api.h"
 #include "src/base/once.h"
@@ skipping 4205 matching lines @@
 
 // ES6 section 25.2.1.1 GeneratorFunction (p1, p2, ... , pn, body)
 BUILTIN(GeneratorFunctionConstructor) {
   HandleScope scope(isolate);
   Handle<JSFunction> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
       isolate, result, CreateDynamicFunction(isolate, args, "function*"));
   return *result;
 }
 
+BUILTIN(AsyncFunctionConstructor) {
+  HandleScope scope(isolate);
+  Handle<JSFunction> result;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, result, CreateDynamicFunction(isolate, args, "async function"));
+  return *result;
+}
 
 // ES6 section 19.4.1.1 Symbol ( [ description ] ) for the [[Call]] case.
 BUILTIN(SymbolConstructor) {
   HandleScope scope(isolate);
   Handle<Symbol> result = isolate->factory()->NewSymbol();
   Handle<Object> description = args.atOrUndefined(isolate, 1);
   if (!description->IsUndefined()) {
     ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, description,
                                        Object::ToString(isolate, description));
     result->set_name(*description);
@@ skipping 1242 matching lines @@
 BUILTIN_LIST_T(DEFINE_BUILTIN_ACCESSOR_T)
 BUILTIN_LIST_H(DEFINE_BUILTIN_ACCESSOR_H)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 #undef DEFINE_BUILTIN_ACCESSOR_T
 #undef DEFINE_BUILTIN_ACCESSOR_H
 
 }  // namespace internal
 }  // namespace v8