Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/hosted/stay_locked_test.dart

 // Copyright (c) 2012, 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 pub_tests;
 
-import 'dart:io';
-
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../../lib/src/io.dart';
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('keeps a pub server package locked to the version in the '
@@ skipping 12 matching lines @@
 
     // Start serving a newer package as well.
     servePackages([packageMap("foo", "1.0.1")]);
 
     // This install shouldn't update the foo dependency due to the lockfile.
     pubInstall();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
   });
 }