Restructure the yaml package.

pkg/yaml/lib/src/constructor.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 constructor;
+
+import 'model.dart';
+import 'visitor.dart';
+import 'yaml_map.dart';
 
 /// Takes a parsed and composed YAML document (what the spec calls the
 /// "representation graph") and creates native Dart objects that represent that
 /// document.
-class _Constructor extends _Visitor {
+class Constructor extends Visitor {
   /// The root node of the representation graph.
-  _Node root;
+  final Node _root;
 
   /// Map from anchor names to the most recent Dart node with that anchor.
-  Map<String, dynamic> anchors;
+  final _anchors = <String, dynamic>{};
 
-  _Constructor(this.root) : this.anchors = {};
+  Constructor(this._root);
 
   /// Runs the Constructor to produce a Dart object.
-  construct() => root.visit(this);
+  construct() => _root.visit(this);
 
   /// Returns the value of a scalar.
-  visitScalar(_ScalarNode scalar) => scalar.value;
+  visitScalar(ScalarNode scalar) => scalar.value;
 
   /// Converts a sequence into a List of Dart objects.
-  visitSequence(_SequenceNode seq) {
+  visitSequence(SequenceNode seq) {
     var anchor = getAnchor(seq);
     if (anchor != null) return anchor;
     var dartSeq = setAnchor(seq, []);
     dartSeq.addAll(super.visitSequence(seq));
     return dartSeq;
   }
 
   /// Converts a mapping into a Map of Dart objects.
-  visitMapping(_MappingNode map) {
+  visitMapping(MappingNode map) {
     var anchor = getAnchor(map);
     if (anchor != null) return anchor;
     var dartMap = setAnchor(map, new YamlMap());
     super.visitMapping(map).forEach((k, v) { dartMap[k] = v; });
     return dartMap;
   }
 
   /// Returns the Dart object that already represents [anchored], if such a
   /// thing exists.
-  getAnchor(_Node anchored) {
+  getAnchor(Node anchored) {
     if (anchored.anchor == null) return null;
-    if (anchors.containsKey(anchored.anchor)) return anchors[anchored.anchor];
+    if (_anchors.containsKey(anchored.anchor)) return _anchors[anchored.anchor];
   }
 
   /// Records that [value] is the Dart object representing [anchored].
-  setAnchor(_Node anchored, value) {
+  setAnchor(Node anchored, value) {
     if (anchored.anchor == null) return value;
-    anchors[anchored.anchor] = value;
+    _anchors[anchored.anchor] = value;
     return value;
   }
 }