[esnext] prototype runtime implementation for async functions

test/mjsunit/harmony/async-arrow-lexical-this.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
+
+var O = {
+  [Symbol.toStringTag]: "LexicalThis",
+  run(n) {
+    return async passFail => `${n}. ${passFail}: ${this}`;
+  },
+};
+
+assertEqualsAsync("1. PASS: [object LexicalThis]", () => O.run(1)("PASS"));
+
+var O2 = {
+  [Symbol.toStringTag]: "LexicalThis",
+  run: O.run(2)
+};
+
+assertEqualsAsync("2. PASS: [object LexicalThis]", () => O2.run("PASS"));