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 709a73d1d1de2d76ab931aa9767175081aaa62ec..9fd0ca1509b815c4e147856aca4979fac497e47b 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -546,11 +546,12 @@ 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 [onTimeout] function must return a [T] or a `Future<T>`.
    *
    * 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()});
 }
 
 /**