Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/broken_symlink_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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   integration('replaces a broken "packages" symlink', () {
     d.dir(appPath, [
       d.appPubspec(),
@@ skipping 33 matching lines @@
       d.dir("bin", [
         d.dir("packages", [
           d.dir("myapp", [
             d.file('foo.dart', 'main() => "foo";')
           ])
         ])
       ])
     ]).validate();
   });
 }