Add SingleSubscriptionTransformer.

test/single_subscription_transformer_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:async/async.dart';
+import 'package:test/test.dart';
+
+import 'utils.dart';
+
+void main() {
+  test("buffers events as soon as it's bound", () async {
+    var controller = new StreamController.broadcast();
+    var stream = controller.stream.transform(
+        const SingleSubscriptionTransformer());
+
+    // Add events before [stream] has a listener to be sure it buffers them.
+    controller.add(1);
+    controller.add(2);
+    await flushMicrotasks();
+
+    expect(stream.toList(), completion(equals([1, 2, 3, 4])));
+    await flushMicrotasks();
+
+    controller.add(3);
+    controller.add(4);
+    controller.close();
+  });
+
+  test("cancels the subscription to the broadcast stream when it's canceled",
+      () async {
+    var canceled = false;
+    var controller = new StreamController.broadcast(onCancel: () {
+      canceled = true;
+    });
+    var stream = controller.stream.transform(
+        const SingleSubscriptionTransformer());
+    await flushMicrotasks();
+    expect(canceled, isFalse);
+
+    var subscription = stream.listen(null);
+    await flushMicrotasks();
+    expect(canceled, isFalse);
+
+    subscription.cancel();
+    await flushMicrotasks();
+    expect(canceled, isTrue);
+  });
+}