[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 --allow-natives-syntax
+
+function assertEqualsAsync(expected, run, msg) {
+  var actual;
+  var hadValue = false;
+  var hadError = false;
+  var promise = run();
+
+  if (typeof promise !== "object" || typeof promise.then !== "function") {
+    throw new MjsUnitAssertionError(
+        "Expected " + run.toString() +
+        " to return a Promise, but it returned " + PrettyPrint(promise));
+  }
+
+  promise.then(function(value) { hadValue = true; actual = value; },
+               function(error) { hadError = true; actual = error; });
+
+  assertFalse(hadValue || hadError);
+
+  %RunMicrotasks();
+
+  if (hadError) throw actual;
+
+  assertTrue(
+      hadValue, "Expected '" + run.toString() + "' to produce a value");
+
+  assertEquals(expected, actual, msg);
+};
+
+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"));