Fix string_transformer.

sdk/lib/async/stream.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;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 996 matching lines @@
   final Stream<S> _source;
   final StreamEventTransformer _transformer;
   EventTransformStream(Stream<S> source,
                        StreamEventTransformer<S, T> transformer)
       : _source = source, _transformer = transformer;
 
   StreamSubscription<T> listen(void onData(T data),
                                { void onError(AsyncError error),
                                  void onDone(),
                                  bool unsubscribeOnError }) {
+    unsubscribeOnError = identical(true, unsubscribeOnError);
     return new _EventTransformStreamSubscription(_source, _transformer,
                                                  onData, onError, onDone,
                                                  unsubscribeOnError);
   }
 }
 
 class _EventTransformStreamSubscription<S, T>
     extends _BaseStreamSubscription<T>
     implements _StreamOutputSink<T> {
   /** The transformer used to transform events. */
@@ skipping 77 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();
 }