Restructure the yaml package.

pkg/yaml/lib/src/visitor.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;
+library visitor;
+
+import 'model.dart';
+import 'yaml_map.dart';
 
 /// The visitor pattern for YAML documents.
-class _Visitor {
+class Visitor {
   /// Returns [alias].
-  visitAlias(_AliasNode alias) => alias;
+  visitAlias(AliasNode alias) => alias;
 
   /// Returns [scalar].
-  visitScalar(_ScalarNode scalar) => scalar;
+  visitScalar(ScalarNode scalar) => scalar;
 
   /// Visits each node in [seq] and returns a list of the results.
-  visitSequence(_SequenceNode seq)
+  visitSequence(SequenceNode seq)
       => seq.content.map((e) => e.visit(this)).toList();
 
   /// Visits each key and value in [map] and returns a map of the results.
-  visitMapping(_MappingNode map) {
+  visitMapping(MappingNode map) {
     var out = new YamlMap();
     for (var key in map.content.keys) {
       out[key.visit(this)] = map.content[key].visit(this);
     }
     return out;
   }
 }