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

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index 68dbbc041e7f49408f1edec2aa481a3cf0805d93..ad72431842b63c20eec4387842c86a1236b2601a 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -680,9 +680,9 @@ class _Future<T> implements Future<T> {
     }
   }
 
-  Future timeout(Duration timeLimit, {onTimeout()}) {
+  Future<T> timeout(Duration timeLimit, {onTimeout()}) {
     if (_isComplete) return new _Future.immediate(this);
-    _Future result = new _Future();
+    _Future result = new _Future<T>();
     Timer timer;
     if (onTimeout == null) {
       timer = new Timer(timeLimit, () {