Make generic type of timeout-future the same as the one from the original.

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 5d6a65009a4afae64ff5d150d7d8816bc876526f..a60a57854ae4c0d99e82b15a820c6ae10a524769 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -521,11 +521,12 @@ abstract class Future<T> {
    * If this future does not complete before `timeLimit` has passed,
    * the [onTimeout] action is executed instead, and its result (whether it
    * returns or throws) is used as the result of the returned future.
+   * The result of the [onTimeout] function must return a [T] or a `Future<T>`.

[Lasse Reichstein Nielsen, 2016/02/09 16:37:05]

either "the [onTimeout] function must return"
or "the result of the [onTimeout] function must be"

[floitsch, 2016/02/09 17:04:21]

Done.

    *
    * If `onTimeout` is omitted, a timeout will cause the returned future to
    * complete with a [TimeoutException].
    */
-  Future timeout(Duration timeLimit, {onTimeout()});
+  Future<T> timeout(Duration timeLimit, {onTimeout()});
 }
 
 /**