[esnext] prototype runtime implementation for async functions

test/mjsunit/harmony/async-arrow-lexical-arguments.js

+// Copyright 2016 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.
+
+// Flags: --harmony-async-await
+
+assertEqualsAsync("[1,2,3]", () => (function() {
+  return (async () => JSON.stringify([...arguments]))();
+})(1, 2, 3));
+
+assertEqualsAsync("[4,5,6]",
+    () => (function() {
+      return (async () => {
+        return JSON.stringify([...await arguments]) })();
+      })(4, 5, 6));