Remove unused imports in pub.

sdk/lib/_internal/pub/test/pub_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';
 
 final USAGE_STRING = """
     Pub is a package manager for Dart.
 
     Usage: pub command [arguments]
 
@@ skipping 152 matching lines @@
       // The "version" file generated on developer builds is a little funky and
       // we need to make sure we don't choke on it.
       d.dir(sdkPath, [
         d.file('version', '0.1.2.0_r16279_bobross'),
       ]).create();
 
       schedulePub(args: ['version'], output: "Pub 0.1.2+0.r16279.bobross\n");
     });
   });
 }