[builtins] make AsyncFunction constructor a subclass of Function

Corrects a small problem with the current implementation of the AsyncFunction
constructor.

See https://tc39.github.io/ecmascript-asyncawait/#async-function-constructor for
details.

BUG=v8:4483
R=littledan@chromium.org, adamk@chromium.org, jwolfe@igalia.com

Review-Url: https://codereview.chromium.org/2118653004
Cr-Commit-Position: refs/heads/master@{#37484}

src/bootstrapper.cc

@@ -2637,14 +2637,14 @@ void Bootstrapper::ExportFromRuntime(Isolate* isolate,
           container, "AsyncFunction", JS_FUNCTION_TYPE, JSFunction::kSize,
           async_function_prototype, Builtins::kAsyncFunctionConstructor,
           kUseStrictFunctionMap);
-      async_function_constructor->set_prototype_or_initial_map(
-          native_context->sloppy_async_function_map());
       async_function_constructor->shared()->DontAdaptArguments();
       async_function_constructor->shared()->set_construct_stub(
           *isolate->builtins()->AsyncFunctionConstructor());
       async_function_constructor->shared()->set_length(1);
       InstallWithIntrinsicDefaultProto(isolate, async_function_constructor,
                                        Context::ASYNC_FUNCTION_FUNCTION_INDEX);
+      SetObjectPrototype(async_function_constructor,
+                         isolate->function_function());
 
       JSObject::AddProperty(
           async_function_prototype, factory->constructor_string(),

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

@@ -70,6 +70,15 @@ function assertEqualsAsync(expected, run, 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,