Rewrite the pkg/yaml parser.

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.
-
-library yaml.visitor;
-
-import 'equality.dart';
-import 'model.dart';
-
-/// 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.map((e) => e.visit(this)).toList();
-
-  /// Visits each key and value in [map] and returns a map of the results.
-  visitMapping(MappingNode map) {
-    var out = deepEqualsMap();
-    for (var key in map.content.keys) {
-      out[key.visit(this)] = map.content[key].visit(this);
-    }
-    return out;
-  }
-}