Remove unused imports in pub.

sdk/lib/_internal/pub/lib/src/command/help.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.command.help;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import '../command.dart';
 import '../exit_codes.dart' as exit_codes;
-import '../io.dart';
 import '../log.dart' as log;
 
 /// Handles the `help` pub command.
 class HelpCommand extends PubCommand {
   String get description => "Display help information for Pub.";
   String get usage => 'pub help [command]';
   bool get requiresEntrypoint => false;
 
   Future onRun() {
     if (commandOptions.rest.isEmpty) {
       printUsage();
     } else {
       var name = commandOptions.rest[0];
       var command = PubCommand.commands[name];
       if (command == null) {
         log.error('Could not find a command named "$name".');
         log.error('Run "pub help" to see available commands.');
         io.exit(exit_codes.USAGE);
       }
 
       command.printUsage();
     }
   }
 }