Remove unused imports in pub.

sdk/lib/_internal/pub/test/update/hosted/unlock_dependers_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 '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration("updates a locked package's dependers in order to get it to max "
       "version", () {
     servePackages([
       packageMap("foo", "1.0.0", {"bar": "<2.0.0"}),
       packageMap("bar", "1.0.0")
@@ skipping 14 matching lines @@
     ]);
 
     pubUpdate(args: ['bar']);
 
     d.packagesDir({
       "foo": "2.0.0",
       "bar": "2.0.0"
     }).validate();
   });
 }