Update YAML to latest language changes.

utils/pub/yaml/model.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.
 
 // This file contains the node classes for the internal representations of YAML
 // documents. These nodes are used for both the serialization tree and the
 // representation graph.
 
 /** A tag that indicates the type of a YAML node. */
 class _Tag {
@@ skipping 32 matching lines @@
       return '!!${name.substring(YAML_URI_PREFIX.length)}';
     } else {
       return '!<$name>';
     }
   }
 
   int get hashCode => name.hashCode;
 }
 
 /** The abstract class for YAML nodes. */
-class _Node {
+abstract class _Node {
   /** Every YAML node has a tag that describes its type. */
   _Tag tag;
 
   /** Any YAML node can have an anchor associated with it. */
   String anchor;
 
   _Node(this.tag, [this.anchor]);
 
   bool operator ==(other) {
     if (other is! _Node) return false;
     return tag == other.tag;
   }
 
   int get hashCode => _hashCode([tag, anchor]);
 
-  abstract visit(_Visitor v);
+  visit(_Visitor v);
 }
 
 /** A sequence node represents an ordered list of nodes. */
 class _SequenceNode extends _Node {
   /** The nodes in the sequence. */
   List<_Node> content;
 
   _SequenceNode(String tagName, this.content)
     : super(new _Tag.sequence(tagName));
 
@@ skipping 149 matching lines @@
   String toString() {
     var strContent = Strings.join(content.keys.
         map((k) => '${k}: ${content[k]}'), ', ');
     return '$tag {$strContent}';
   }
 
   int get hashCode => super.hashCode ^ _hashCode(content);
 
   visit(_Visitor v) => v.visitMapping(this);
 }