Remove unused imports in pub.

sdk/lib/_internal/pub/test/pub_install_and_update_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_test.dart';
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubInstallAndUpdate((command) {
     group('requires', () {
@@ skipping 91 matching lines @@
           "foo": {"path": "../deps/foo"},
           "bar": {"path": "../deps/bar"}
         })
       ]).create();
 
       pubCommand(command,
           error: new RegExp("^Incompatible dependencies on 'baz':\n"));
     });
   });
 }