Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart

Index: pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart b/pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart
index cad599fcec667731a51104a72b4159b5a32a59f1..322e17fcb821792134503b07cfc188d3149b5d46 100644
--- a/pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/wrap_async_test.dart
@@ -3,7 +3,7 @@
 // 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;
@@ -34,4 +34,4 @@ void main() {
       schedule(() => sleep(1));
     });
   });
-}
+}