Avoid crashing when a closed StreamIterator is cancelled

sdk/lib/async/stream_impl.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;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error, StackTrace stackTrace);
@@ skipping 1011 matching lines @@
   /** Clears up the internal state when the iterator ends. */
   void _clear() {
     _subscription = null;
     _futureOrPrefetch = null;
     _current = null;
     _state = _STATE_DONE;
   }
 
   Future cancel() {
     StreamSubscription subscription = _subscription;
+    if (subscription == null) return null;
     if (_state == _STATE_MOVING) {
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
     } else {
       _clear();
     }
     return subscription.cancel();
   }
 
@@ skipping 31 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }