Make _SingleStreamMultiplexer accept any Stream as source.

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;
 
 // States shared by single/multi stream implementations.
 
 /// Initial and default state where the stream can receive and send events.
 const int _STREAM_OPEN = 0;
@@ skipping 1168 matching lines @@
     }
     if (_timer != null) {
       _timer.cancel();
       _timer = null;
     }
     _pauseCount = 0;
   }
 }
 
 class _SingleStreamMultiplexer<T> extends _MultiStreamImpl<T> {
-  final _SingleStreamImpl<T> _source;
+  final Stream<T> _source;
   StreamSubscription<T> _subscription;
 
   _SingleStreamMultiplexer(this._source);
 
   void _callOnPauseStateChange() {
     if (_isPaused) {
       if (_subscription != null) {
         _subscription.pause();
       }
     } else {
@@ skipping 14 matching lines @@
                                      onError: this._signalError,
                                      onDone: this._close);
     } else {
       // TODO(lrn): Check why this can happen.
       if (_subscription == null) return;
       _subscription.cancel();
       _subscription = null;
     }
   }
 }