[esnext] Async function toString support

test/mjsunit/harmony/async-await-basic.js

Index: test/mjsunit/harmony/async-await-basic.js
diff --git a/test/mjsunit/harmony/async-await-basic.js b/test/mjsunit/harmony/async-await-basic.js
index 9c8af3c85e388ba64d9e81c28ca3ec85aa3348bb..d0888eaf33ffca6869152e1f4444188a55f61b7f 100644
--- a/test/mjsunit/harmony/async-await-basic.js
+++ b/test/mjsunit/harmony/async-await-basic.js
@@ -336,3 +336,12 @@ var resumeAfterThrowNewEval = new AsyncFunction("value", `
 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));