Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/path/relative_symlink_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 
-import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   // Pub uses NTFS junction points to create links in the packages directory.
   // These (unlike the symlinks that are supported in Vista and later) do not
   // support relative paths. So this test, by design, will not pass on Windows.
   // So just skip it.
   if (Platform.operatingSystem == "windows") return;
 
@@ skipping 22 matching lines @@
 
     d.dir("moved", [
       d.dir(packagesPath, [
         d.dir("foo", [
           d.file("foo.dart", 'main() => "foo";')
         ])
       ])
     ]).validate();
   });
 }