Factor failing tests out of language/async_star_test.dart

tests/language/async_star_regression_2238_test.dart

 // Copyright (c) 2015, 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.
 
-// Test that an async function does not start immediately.
-
+// This is a regression test for issue 2238
+import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
-var x = 0;
+main() {
+  f() async* {
+    label1: label2: yield 0;
+  }
 
-foo() async {
-  x++;
-  await 1;
-  x++;
+  asyncStart();
+  f().toList().then((list) {
+    Expect.listEquals([0], list);
+    asyncEnd();
+  });
 }
-
-void main() {
-  asyncStart();
-  foo().then((_) => Expect.equals(2, x)).whenComplete(asyncEnd);
-  Expect.equals(0, x);
-}