Factor failing tests out of language/async_star_test.dart

tests/language/async_star_cancel_while_paused_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.
+
+// This is a regression test for issue 22853.
+
+import "dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+main() {
+  var list = [];
+  var sync = new Sync();
+  f() async* {
+    list.add("*1");
+    yield 1;
+    await sync.wait();
+    sync.release();
+    list.add("*2");
+    yield 2;
+    list.add("*3");
+  };
+  var stream = f();
+  var sub = stream.listen(list.add);
+
+  asyncStart();
+  return sync.wait().whenComplete(() {
+    Expect.listEquals(list, ["*1", 1]);
+    sub.pause();
+    return sync.wait();
+  }).whenComplete(() {
+    Expect.listEquals(list, ["*1", 1, "*2"]);
+    sub.cancel();
+    new Future.delayed(new Duration(milliseconds: 200), () {
+      // Should not have yielded 2 or added *3 while paused.
+      Expect.listEquals(list, ["*1", 1, "*2"]);
+      asyncEnd();
+    });
+  });
+}
+
+/**
+ * Allows two asynchronous executions to synchronize.
+ *
+ * Calling [wait] and waiting for the returned future to complete will
+ * wait for the other executions to call [wait] again. At that point,
+ * the waiting execution is allowed to continue (the returned future completes),
+ * and the more recent call to [wait] is now the waiting execution.
+ */
+class Sync {
+  Completer _completer = null;
+  // Release whoever is currently waiting and start waiting yourself.
+  Future wait([v]) {
+    if (_completer != null) _completer.complete(v);
+    _completer = new Completer();
+    return _completer.future;
+  }
+
+  // Release whoever is currently waiting.
+  void release([v]) {
+    if (_completer != null) {
+      _completer.complete(v);
+      _completer = null;
+    }
+  }
+}