Remove unused imports in pub.

sdk/lib/_internal/pub/bin/pub.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.
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:args/args.dart';
 import 'package:path/path.dart' as path;
 
 import '../lib/src/command.dart';
 import '../lib/src/exit_codes.dart' as exit_codes;
 import '../lib/src/io.dart';
 import '../lib/src/log.dart' as log;
 import '../lib/src/sdk.dart' as sdk;
-import '../lib/src/system_cache.dart';
 import '../lib/src/utils.dart';
 
 final pubArgParser = initArgParser();
 
 void main() {
   ArgResults options;
 
   try {
     options = pubArgParser.parse(new Options().arguments);
   } on FormatException catch (e) {
@@ skipping 133 matching lines @@
   for (var name in names) {
     buffer.write('  ${padRight(name, length)}   '
         '${PubCommand.commands[name].description}\n');
   }
 
   buffer.write('\n');
   buffer.write(
       'Use "pub help [command]" for more information about a command.');
   log.message(buffer.toString());
 }