Move Yaml into utils/pub.

utils/pub/yaml/yaml.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");
 
 #import("deep_equals.dart");
 
 #source("yaml_map.dart");
 #source("model.dart");
@@ skipping 35 matching lines @@
 }
 
 /** An error thrown by the YAML processor. */
 class YamlException implements Exception {
   String msg;
 
   YamlException(this.msg);
 
   String toString() => msg;
 }