Fix a synchronous event bug in futureStream.

pkg/barback/lib/src/utils.dart

Index: pkg/barback/lib/src/utils.dart
diff --git a/pkg/barback/lib/src/utils.dart b/pkg/barback/lib/src/utils.dart
index 51d98eef0b0b0aa89611bfc4ebaa2edcad23fd81..d180137a69544a64035c0da567797df28f426949 100644
--- a/pkg/barback/lib/src/utils.dart
+++ b/pkg/barback/lib/src/utils.dart
@@ -194,9 +194,10 @@ Stream futureStream(Future<Stream> future, {bool broadcast: false}) {
   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;
   });