Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/lib/src/lock_file.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 lock_file;
 
 import 'dart:json' as json;
 
 import 'package:yaml/yaml.dart';
 
@@ skipping 88 matching lines @@
 
     // TODO(nweiz): Serialize using the YAML library once it supports
     // serialization. For now, we use JSON, since it's a subset of YAML anyway.
     return
         '# Generated by pub\n'
         '# See http://pub.dartlang.org/doc/glossary.html#lockfile\n'
         '\n'
         '${json.stringify({'packages': packagesObj})}\n';
   }
 }