Fix a synchronous event bug in futureStream.

pkg/barback/lib/src/utils.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.utils;
 
 import 'dart:async';
 
 /// A pair of values.
 class Pair<E, F> {
@@ skipping 176 matching lines @@
 /// then close.
 ///
 /// If [broadcast] is true, a broadcast stream is returned. This assumes that
 /// the stream returned by [future] will be a broadcast stream as well.
 /// [broadcast] defaults to false.
 Stream futureStream(Future<Stream> future, {bool broadcast: false}) {
   var subscription;
   var controller;
 
   future = future.catchError((e, stackTrace) {
-    if (controller == null) return;
-    controller.addError(e, stackTrace);
-    controller.close();
+    // Since [controller] is synchronous, it's likely that emitting an error
+    // will cause it to be cancelled before we call close.
+    if (controller != null) controller.addError(e, stackTrace);
+    if (controller != null) controller.close();
     controller = null;
   });
 
   onListen() {
     future.then((stream) {
       if (controller == null) return;
       subscription = stream.listen(
           controller.add,
           onError: controller.addError,
           onDone: controller.close);
@@ skipping 27 matching lines @@
     subscription = callback().listen(controller.add,
         onError: controller.addError,
         onDone: controller.close);
   },
       onCancel: () => subscription.cancel(),
       onPause: () => subscription.pause(),
       onResume: () => subscription.resume(),
       sync: true);
   return controller.stream;
 }