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

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 0c4daa0895577b9b0049396ad92f6ee1cecb523f..f2d926440737fa0f0e519f45eace8a395d21bd4c 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -113,7 +113,7 @@ abstract class Future<T> {
    * 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;
   }