Restructure YAML package suitable for pub lish

pkg/yaml/lib/model.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.
 
 part of yaml;
 
 // This file contains the node classes for the internal representations of YAML
 // documents. These nodes are used for both the serialization tree and the
 // representation graph.
 
@@ skipping 214 matching lines @@
     var strContent = content.keys
         .mappedBy((k) => '${k}: ${content[k]}')
         .join(', ');
     return '$tag {$strContent}';
   }
 
   int get hashCode => super.hashCode ^ _hashCode(content);
 
   visit(_Visitor v) => v.visitMapping(this);
 }