Add Stream.empty.

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 1063 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }
+
+/** An empty broadcast stream, sending a done event as soon as possible. */
+class _EmptyStream<T> extends Stream<T> {
+  const _EmptyStream() : super._internal();
+  bool get isBroadcast => true;
+  StreamSubscription<T> listen(void onData(T data),
+                               {Function onError,
+                                void onDone(),
+                                bool cancelOnError}) {
+    return new _DoneStreamSubscription<T>(onDone);
+  }
+}