Migrate block 39 language/async_backwards ... /async_star_take.

tests/language/async_star_no_cancel2_test.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";
-import "package:expect/expect.dart";
-import "package:async_helper/async_helper.dart";
-
-var events = [];
-
-var timer;
-ticker(period) async* {
-  var sc;
-  sc = new StreamController(onListen: () {
-    events.add("listen");
-    timer = new Timer.periodic(period, (_) {
-      sc.add(null);
-    });
-  }, onCancel: () {
-    events.add("cancel");
-    timer.cancel();
-  });
-
-  try {
-    var counter = 0;
-    await for (var tick in sc.stream) {
-      counter++;
-    }
-  } finally {
-    events.add("finally");
-  }
-}
-
-void main() {
-  asyncStart();
-  events.add("main");
-  final subscription =
-      ticker(const Duration(milliseconds: 20)).listen((val) {});
-
-  bool cancelFinished = false;
-  new Timer(const Duration(milliseconds: 100), () async {
-    // Despite the cancel call below, the stream doesn't stop.
-    // The async* function is not blocked at any await (since the inner timer
-    // continuously ticks), but since there/ is no yield-point in the function
-    // it won't cancel.
-    new Timer(const Duration(milliseconds: 30), () {
-      Expect.isFalse(cancelFinished);
-      Expect.listEquals(["main", "listen", "invoke cancel"], events);
-      timer.cancel();
-      asyncEnd();
-    });
-
-    events.add("invoke cancel");
-    await subscription.cancel();
-    // This line should never be reached, since the cancel-future doesn't
-    // complete.
-    cancelFinished = true;
-  });
-}