Fix warnings.

sdk/lib/_internal/pub/test/hosted/version_negotiation_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:scheduled_test/scheduled_server.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubInstallAndUpdate((command) {
@@ skipping 44 matching lines @@
       });
 
       pub.shouldExit(1);
 
       expect(pub.remainingStderr(), completion(equals(
           "Pub 0.1.2+3 is incompatible with the current version of 127.0.0.1.\n"
           "Upgrade pub to the latest version and try again.")));
     });
   });
 }