Remove unused imports in pub.

sdk/lib/_internal/pub/test/hosted/offline_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();
 
   forBothPubInstallAndUpdate((command) {
     integration('updates a package using the cache', () {
       // Run the server so that we know what URL to use in the system cache.
       servePackages([]);
@@ skipping 41 matching lines @@
       }, includePubspecs: true).create();
 
       d.appDir({"foo": ">2.0.0"}).create();
 
       pubCommand(command, args: ['--offline'], error:
           "Package 'foo' has no versions that match >2.0.0 derived from:\n"
           "- 'myapp' depends on version >2.0.0");
     });
   });
 }