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..be40aa41c9d4543f71d66cc87e55ae2d486953cd 100644
--- a/lib/src/stream_queue.dart
+++ b/lib/src/stream_queue.dart
@@ -236,14 +236,15 @@ class StreamQueue<T> {
     if (_isClosed) throw _failClosed();
     _isClosed = true;
 
+    if (_isDone) return new Future.value();
+    if (_subscription == null) _subscription = _sourceStream.listen(null);
+
     if (!immediate) {
       var request = new _CancelRequest(this);
       _addRequest(request);
       return request.future;

[Lasse Reichstein Nielsen, 2015/07/14 07:28:10]

That doesn't look right either - you subscribe to the stream, but don't set up
the callbacks. For a non-immediate cancel, that would mean it won't get the
onDone event, so the _CancelRequest never completes.

On the other hand, if you haven't subscribed yet, then it means that the cancel
is the first request, so the _eventQueue is empty.
In that case, you can just cancel it immediately, no need to enqueue anything.
So maybe change the enqueue-event case condition to:
 if (_isDone) return new Future.value();
 if (!immediate && _eventQueue.isNotEmpty) {
    assert(_subscription != null);
    ...
 }
 if (_subscription == null) _subscription = _sourceStream.listen(null);
 ...

Alternatively, andprobably better, fix this at the root cause: That a
_CancelRequest doesn't listen before it cancels if there is no subscription. See
below.

     }
 
-    if (_isDone) return new Future.value();
-    if (_subscription == null) _subscription = _sourceStream.listen(null);
     var future = _subscription.cancel();
     _onDone();
     return future;