[esnext] Async function toString support

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 318 matching lines @@
     }
     try {
       value = await rejectLater("throw2");
     } catch (e) {
       log.push("resume:" + e);
     }
     return value + 1;`);
 log = [];
 assertEqualsAsync(6, () => resumeAfterThrowNewEval(5));
 assertEquals("start:5 resume:throw1 resume:throw2", log.join(" "));
+
+async function foo() {}
+assertEquals("async function foo() {}", foo.toString());
+assertEquals("async function () {}", async function() {}.toString());
+assertEquals("async x => x", (async x => x).toString());
+assertEquals("async x => { return x }", (async x => { return x }).toString());
+class AsyncMethod { async foo() { } }
+assertEquals("async foo() { }", Function.prototype.toString.call(AsyncMethod.prototype.foo));
+assertEquals("async foo() { }", Function.prototype.toString.call({async foo() { }}.foo));