Clean-up of Future documentation and small fix-ups.

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 978 matching lines @@
     assert(_subscription == null);
     var stateData = _stateData;
     if (stateData != null) {
       Stream<T> stream = stateData as Object/*=Stream<T>*/;
       _subscription = stream.listen(_onData,
           onError: _onError, onDone: _onDone, cancelOnError: true);
       var future = new _Future<bool>();
       _stateData = future;
       return future;
     }
-    return new _Future<bool>.immediate(false);
+    return Future._falseFuture;
   }
 
   Future cancel() {
     StreamSubscription<T> subscription = _subscription;
     Object stateData = _stateData;
     _stateData = null;
     if (subscription != null) {
       _subscription = null;
       if (!_isPaused) {
         _Future<bool> future = stateData as Object/*=_Future<bool>*/;
@@ skipping 32 matching lines @@
 
 /** 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);
   }
 }