[esnext] prototype runtime implementation for async functions

test/mjsunit/harmony/async-arrow-lexical-new.target.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
+
+class BaseClass {
+  constructor() {
+    return async () => new.target;
+  }
+}
+
+class ChildClass extends BaseClass {}
+
+assertEqualsAsync(BaseClass, () => new BaseClass()());
+assertEqualsAsync(ChildClass, () => new ChildClass()());