[builtins] make AsyncFunction constructor a subclass of Function

test/mjsunit/harmony/async-await-basic.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
 
 // Do not install `AsyncFunction` constructor on global object
 
 function assertThrowsAsync(run, errorType, message) {
   var actual;
@@ skipping 52 matching lines @@
 
   assertTrue(
       hadValue, "Expected '" + run.toString() + "' to produce a value");
 
   assertEquals(expected, actual, msg);
 };
 
 assertEquals(undefined, this.AsyncFunction);
 let AsyncFunction = (async function() {}).constructor;
 
+// The AsyncFunction Constructor is the %AsyncFunction% intrinsic object and
+// is a subclass of Function.
+// (https://tc39.github.io/ecmascript-asyncawait/#async-function-constructor)
+assertEquals(Object.getPrototypeOf(AsyncFunction), Function);
+assertEquals(Object.getPrototypeOf(AsyncFunction.prototype),
+             Function.prototype);
+assertTrue(async function() {} instanceof Function);
+
+
 // Let functionPrototype be the intrinsic object %AsyncFunctionPrototype%.
 async function asyncFunctionForProto() {}
 assertEquals(AsyncFunction.prototype,
              Object.getPrototypeOf(asyncFunctionForProto));
 assertEquals(AsyncFunction.prototype,
              Object.getPrototypeOf(async function() {}));
 assertEquals(AsyncFunction.prototype, Object.getPrototypeOf(async () => {}));
 assertEquals(AsyncFunction.prototype,
              Object.getPrototypeOf({ async method() {} }.method));
 assertEquals(AsyncFunction.prototype, Object.getPrototypeOf(AsyncFunction()));
@@ skipping 270 matching lines @@
 
 // Regress v8:5148
 assertEqualsAsync("1", () => (async({ a = NaN }) => a)({ a: "1" }));
 assertEqualsAsync(
     "10", () => (async(foo, { a = NaN }) => foo + a)("1", { a: "0" }));
 assertEqualsAsync("2", () => (async({ a = "2" }) => a)({ a: undefined }));
 assertEqualsAsync(
     "20", () => (async(foo, { a = "0" }) => foo + a)("2", { a: undefined }));
 assertThrows(() => eval("async({ foo = 1 })"), SyntaxError);
 assertThrows(() => eval("async(a, { foo = 1 })"), SyntaxError);