Fix a bug in StreamQueue.cancel().

lib/src/stream_queue.dart

Index: lib/src/stream_queue.dart
diff --git a/lib/src/stream_queue.dart b/lib/src/stream_queue.dart
index 7d78ac593c79181bb504897f8f3b611130a3b48b..e4b3b640cd8dd14b7ef5e9d72353275dfada3f4a 100644
--- a/lib/src/stream_queue.dart
+++ b/lib/src/stream_queue.dart
@@ -530,9 +530,10 @@ class _CancelRequest implements _EventRequest {
   }
 
   void _shutdown() {
-    if (_streamQueue._subscription == null) {
+    if (_streamQueue._isDone) {
       _completer.complete();
     } else {
+      _streamQueue._ensureListening();
       _completer.complete(_streamQueue._dispose().cancel());
     }
   }