Move pub into sdk/lib/_internal.

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 command_help;
 
 import 'dart:async';
 import 'dart:io' as io;
 import 'exit_codes.dart' as exit_codes;
 import 'io.dart';
@@ skipping 15 matching lines @@
       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();
     }
   }
 }