Remove unused imports in pub.

sdk/lib/_internal/pub/test/pub_cache_test.dart

 // Copyright (c) 2013, 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_cache_test;
 
-import 'dart:io';
-import 'dart:json' as json;
-
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../lib/src/io.dart';
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   integration('running pub cache displays error message', () {
     schedulePub(args: ['cache'],
         output: '''
           Inspect the system cache.
@@ skipping 66 matching lines @@
           ])
         ])
       ])
     ]).create();
 
     schedulePub(args: ['cache', 'list'], output:
       new RegExp(r'\{"packages":\{"foo":\{"1\.2\.3":\{"location":'
           r'"[^"]+foo-1\.2\.3"\}\}\}\}$'));
   });
 }