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

tests/lib/async/future_timeout_test.dart

 // Copyright (c) 2011, 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.
 
 library future_timeout_test;
 
 import 'dart:async';
 import 'package:unittest/unittest.dart';
 
 main() {
@@ skipping 167 matching lines @@
   test("timeoutNoFunction", () {
     Completer completer = new Completer();
     Future timedOut = completer.future.timeout(
         const Duration(milliseconds: 5));
     timedOut.catchError(expectAsync((e, s) {
       expect(e, new isInstanceOf<TimeoutException>());
       expect(e.duration, const Duration(milliseconds: 5));
       expect(s, null);
     }));
   });
+
+  test("timeoutType", () {
+    Completer completer = new Completer<int>();
+    Future timedOut = completer.future.timeout(
+        const Duration(milliseconds: 5));
+    expect(timedOut, new isInstanceOf<Future<int>>());
+    expect(timedOut, isNot(new isInstanceOf<Future<String>>()));
+    timedOut.catchError((_) {});
+    completer.complete(499);
+  });
 }