Restructure YAML package suitable for pub lish

utils/pub/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.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
-import 'yaml/yaml.dart';
+import '../../pkg/yaml/lib/yaml.dart';
 
 /// A parsed and validated `pubspec.lock` file.
 class LockFile {
   /// The packages this lockfile pins.
   Map<String, PackageId> packages;
 
   LockFile._(this.packages);
 
   LockFile.empty()
     : packages = <String, PackageId>{};
@@ skipping 58 matching lines @@
         'source': id.source.name,
         'description': id.description
       };
     });
 
     // 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 json.stringify({'packages': packagesObj});
   }
 }