Reapply "Add a sleep function to dart:io"

pkg/scheduled_test/test/scheduled_test/timeout_test.dart

Index: pkg/scheduled_test/test/scheduled_test/timeout_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/timeout_test.dart b/pkg/scheduled_test/test/scheduled_test/timeout_test.dart
index 62095beb1348a903c2a6cbe4dd7fecb5749018d2..ffcf32f2198ae1fb1198ff051aca4f8b72bf2e0c 100644
--- a/pkg/scheduled_test/test/scheduled_test/timeout_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/timeout_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;
@@ -199,4 +199,4 @@ void main() {
 
   // TODO(nweiz): test out-of-band post-timeout errors that are detected after
   // the test finishes once we can detect top-level errors (issue 8417).
-}
+}