More tests for async* functions

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, since there
+    // is no yield-point where it can cancel.
+    new Timer(const Duration(milliseconds: 30), () {
+      Expect.isFalse(cancelFinished);
+      Expect.listEquals(["main", "listen"], events);
+      timer.cancel();
+      asyncEnd();
+    });
+
+    await subscription.cancel();
+    // This line should never be reached, since the cancel-future doesn't
+    // complete.
+    cancelFinished = true;
+  });
+}