Avoid sending needless data between isolates in "pub serve" and "pub build"

pkg/barback/test/stream_replayer_test.dart

 // Copyright (c) 2013, 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.
 
 library barback.test.stream_replayer_test;
 
 import 'dart:async';
 
 import 'package:barback/src/stream_replayer.dart';
 import 'package:barback/src/utils.dart';
@@ skipping 58 matching lines @@
     controller.add(3);
 
     expect(pumpEventQueue().then((_) {
       expect(isClosed, isFalse);
       controller.close();
       return pumpEventQueue();
     }).then((_) {
       expect(isClosed, isTrue);
     }), completes);
   });
+
+  test("the wrapped stream isn't opened if there are no replays", () {
+    var isOpened = false;
+    var controller = new StreamController<int>(onListen: () {
+      isOpened = true;
+    });
+    var replayer = new StreamReplayer<int>(controller.stream);
+
+    expect(pumpEventQueue().then((_) => isOpened), completion(isFalse));
+  });
+
+  test("the wrapped stream isn't opened if no replays are opened", () {
+    var isOpened = false;
+    var controller = new StreamController<int>(onListen: () {
+      isOpened = true;
+    });
+    var replayer = new StreamReplayer<int>(controller.stream);
+    replayer.getReplay();
+    replayer.getReplay();
+
+    expect(pumpEventQueue().then((_) => isOpened), completion(isFalse));
+  });
 }