Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:async';
-import 'dart:io';
+import 'dart:io' hide sleep;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/src/mock_clock.dart' as mock_clock;
 
 import '../metatest.dart';
 import '../utils.dart';
 
 void main() {
   setUpTimeout();
 
@@ skipping 10 matching lines @@
   expectTestsFail('an out-of-band failure in wrapAsync that finishes after the '
       'schedule is handled', () {
     mock_clock.mock().run();
     test('test', () {
       schedule(() {
         sleep(2).then(wrapAsync((_) => expect('foo', equals('bar'))));
       });
       schedule(() => sleep(1));
     });
   });
-}
+}