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

lib/yaml/visitor.dart

Index: lib/yaml/visitor.dart
diff --git a/lib/yaml/visitor.dart b/lib/yaml/visitor.dart
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index 0000000000000000000000000000000000000000..744bcd7388e7be1f74170f06ce1b62f043db56a3
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+++ b/lib/yaml/visitor.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.
+
+/** 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));
+
+  /** 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.getKeys()) {
+      out[key.visit(this)] = map.content[key].visit(this);
+    }
+    return out;
+  }
+}