Fix expected return type in async functions

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();
+}