Restructure YAML package suitable for pub lish

utils/pub/yaml/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;
-
-/// The visitor pattern for YAML documents.
-class _Visitor {
-  /// Returns [alias].
-  visitAlias(_AliasNode alias) => alias;
-
-  /// Returns [scalar].
-  visitScalar(_ScalarNode scalar) => scalar;
-
-  /// Visits each node in [seq] and returns a list of the results.
-  visitSequence(_SequenceNode seq)
-      => seq.content.mappedBy((e) => e.visit(this)).toList();
-
-  /// Visits each key and value in [map] and returns a map of the results.
-  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;
-  }
-}