Fix for issue 8311.

sdk/lib/async/stream.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.
 
 part of dart.async;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 1060 matching lines @@
 
   void _handleError(AsyncError error) {
     try {
       _transformer.handleError(error, _sink);
     } catch (e, s) {
       _sendError(_asyncError(e, s, error));
     }
   }
 
   void _handleDone() {
+    _subscription = null;
     try {
       _transformer.handleDone(_sink);
     } catch (e, s) {
       _sendError(_asyncError(e, s));
     }
   }
 
   // StreamOutputSink interface.
   void _sendData(T data) {
     _onData(data);
@@ skipping 14 matching lines @@
 }
 
 class _StreamOutputSinkWrapper<T> implements StreamSink<T> {
   _StreamOutputSink _sink;
   _StreamOutputSinkWrapper(this._sink);
 
   void add(T data) => _sink._sendData(data);
   void signalError(AsyncError error) => _sink._sendError(error);
   void close() => _sink._sendDone();
 }