Move kernel baseline tests to front_end.

pkg/front_end/testcases/async_function.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 import 'dart:async';
 
 Future<String> asyncString() async {
   return "foo";
 }
 
 Future<String> asyncString2() async {
@@ skipping 18 matching lines @@
 
 Stream<String> asyncStarString2() async* {
   yield "bar";
 }
 
 List<String> stringList = ["bar"];
 
 main() async {
   String str = await asyncString();
 }