Add a basic YAML processor. Much of the language is still unimplemented.

lib/yaml/constructor.dart

Index: lib/yaml/constructor.dart
diff --git a/lib/yaml/constructor.dart b/lib/yaml/constructor.dart
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+++ b/lib/yaml/constructor.dart
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+// 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.
+
+/**
+ * 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 {
+  /** The root node of the representation graph. */
+  _Node _root;
+
+  /** Map from anchor names to the most recent Dart node with that anchor. */
+  Map<String, Dynamic> _anchors;
+
+  _Constructor(_Node this._root) : this._anchors = {};
+
+  /** Runs the Constructor to produce a Dart object. */
+  construct() => _root.visit(this);
+
+  /** Returns the value of a scalar. */
+  visitScalar(_ScalarNode scalar) => scalar.value;
+
+  /** Converts a sequence into a List of Dart objects. */
+  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) {
+    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) {
+    if (anchored.anchor == null) return null;
+    if (_anchors.containsKey(anchored.anchor)) return _anchors[anchored.anchor];
+  }
+
+  /** Records that [value] is the Dart object representing [anchored]. */
+  _setAnchor(_Node anchored, value) {
+    if (anchored.anchor == null) return value;
+    _anchors[anchored.anchor] = value;
+    return value;
+  }
+}