Add new constructors to YamlNode subclasses.

pkg/yaml/lib/yaml.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 yaml;
 
 import 'src/composer.dart';
 import 'src/constructor.dart';
 import 'src/parser.dart';
 import 'src/yaml_exception.dart';
@@ skipping 51 matching lines @@
   var pair;
   try {
     pair = new Parser(yaml, sourceName).l_yamlStream();
   } on FormatException catch (error) {
     throw new YamlException(error.toString());
   }
 
   var nodes = pair.first
       .map((doc) => new Constructor(new Composer(doc).compose()).construct())
       .toList();
-  return new YamlList(nodes, pair.last);
+  return new YamlList.internal(nodes, pair.last);
 }