Make StreamSubscription be the active part of a stream.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index dec363306776c1f0c380a365b476abd655bd748f..091eaeaf3c7a1cd574bdd0a367c404f0708309e3 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -548,7 +548,9 @@ class _HttpOutboundConsumer implements StreamConsumer {
 
   void _cancel() {
     if (_subscription != null) {
-      _subscription.cancel();
+      StreamSubscription subscription = _subscription;
+      _subscription = null;
+      subscription.cancel();
     }
   }