Factor failing tests out of language/async_star_test.dart

tests/language/async_star_take_reyield_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.
 
+import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
-range(start, end) async* {
-  for (int i = start; i < end; i++) {
-    yield i;
-  }
+expectList(stream, list) {
+  return stream.toList().then((v) {
+    Expect.listEquals(v, list);
+  });
 }
 
-concat(a, b) async* {
-  yield* a;
-  yield* b;
-}
-
-test() async {
-  Expect.listEquals([1, 2, 3, 11, 12, 13],
-                    await concat(range(1, 4), range(11, 14)).toList());
+mkStream(int n) async* {
+  for (int i = 0; i < n; i++) yield i;
 }
 
 main() {
+  f(s) async* {
+    var r = 0;
+    await for(var v in s.take(5)) yield r += v;
+  }
   asyncStart();
-  test().then((_) {
-    asyncEnd();
-  });
-}
+  expectList(f(mkStream(10)), [0, 1, 3, 6, 10]).then(asyncSuccess);
+}