Add public-facing method and class that allows intercepting stream events.

sdk/lib/async/future.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;
 
 /**
  * A [Future] represents a delayed computation. It is used to obtain a not-yet
  * available value, or error, sometime in the future.  Receivers of a
  * [Future] can register callbacks that handle the value or error once it is
@@ skipping 95 matching lines @@
    * The future will be completed after [milliseconds] have passed with
    * the result of calling [value]. If [milliseconds] is 0, it completes at the
    * earliest in the next event-loop iteration.
    *
    * If calling [value] throws, the created future will complete with the
    * error.
    *
    * See [Completer]s, for futures with values that are computed asynchronously.
    */
   factory Future.delayed(int milliseconds, T value()) {
-    _FutureImpl<T> future = new _ThenFuture<dynamic, T>((_) => value());
+    _ThenFuture<dynamic, T> future = new _ThenFuture<dynamic, T>((_) => value());
     new Timer(milliseconds, (_) => future._sendValue(null));
     return future;
   }
 
   /**
    * Wait for all the given futures to complete and collect their values.
    *
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an error,
    * the resulting future also completes with an error. Otherwise the value
@@ skipping 181 matching lines @@
    * The argument [exception] should not be `null`.
    *
    * If [exception] is an [AsyncError], it is used directly as the error
    * message sent to the future's listeners, and [stackTrace] is ignored.
    *
    * Otherwise the [exception] and an optional [stackTrace] is combined into an
    * [AsyncError] and sent to this future's listeners.
    */
   void completeError(Object exception, [Object stackTrace]);
 }