Remove unused imports in pub.

sdk/lib/_internal/pub/test/package_files_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 lock_file_test;
 
-import 'dart:io';
-
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../lib/src/entrypoint.dart';
 import '../lib/src/io.dart';
 import '../lib/src/system_cache.dart';
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 var root;
@@ skipping 196 matching lines @@
             completion(unorderedEquals([
           path.join(root, 'packages', 'subfile1.txt'),
           path.join(root, 'packages', 'subfile2.txt'),
           path.join(root, 'packages', 'subsubdir', 'subsubfile1.txt'),
           path.join(root, 'packages', 'subsubdir', 'subsubfile2.txt')
         ])));
       });
     });
   });
 }