Fix bad handling of done events in _ForwardingStream.

sdk/lib/async/stream_pipe.dart

 // Copyright (c) 2012, 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;
 
 /** Utility function to create an [AsyncError] if [error] isn't one already. */
 AsyncError _asyncError(Object error, Object stackTrace, [AsyncError cause]) {
   if (error is AsyncError) return error;
   if (cause == null) return new AsyncError(error, stackTrace);
@@ skipping 197 matching lines @@
   // checked mode. http://dartbug.com/7733
   void _handleData(/*S*/ data) {
     _stream._handleData(data, this);
   }
 
   void _handleError(AsyncError error) {
     _stream._handleError(error, this);
   }
 
   void _handleDone() {
+    // On a done-event, we have already been unsubscribed.
+    _subscription = null;
     _stream._handleDone(this);
   }
 }
 
 // -------------------------------------------------------------------
 // Stream transformers used by the default Stream implementation.
 // -------------------------------------------------------------------
 
 typedef bool _Predicate<T>(T value);
 
@@ skipping 297 matching lines @@
 
   void handleError(AsyncError error, StreamSink<T> sink) {
     _handleError(error, sink);
   }
 
   void handleDone(StreamSink<T> sink) {
     _handleDone(sink);
   }
 }