Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/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 {
-  return asyncString();
-}
-
-Iterable<String> syncStarString() sync* {
-  yield "foo";
-  yield* syncStarString2();
-  yield* stringList;
-}
-
-Iterable<String> syncStarString2() sync* {
-  yield "foo";
-}
-
-Stream<String> asyncStarString() async* {
-  yield "foo";
-  yield* asyncStarString2();
-  yield await asyncString();
-}
-
-Stream<String> asyncStarString2() async* {
-  yield "bar";
-}
-
-List<String> stringList = ["bar"];
-
-main() async {
-  String str = await asyncString();
-}