Restructure YAML package suitable for pub lish

pkg/yaml/lib/composer.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;
 
 /// Takes a parsed YAML document (what the spec calls the "serialization tree")
 /// and resolves aliases, resolves tags, and parses scalars to produce the
 /// "representation graph".
 class _Composer extends _Visitor {
@@ skipping 153 matching lines @@
           value: double.parse("NaN"));
     }
 
     return null;
   }
 
   /// Parses a string scalar.
   _ScalarNode parseString(String content) =>
     new _ScalarNode(_Tag.yaml("str"), value: content);
 }