Update YAML to latest language changes.

utils/pub/yaml/model.dart

Index: utils/pub/yaml/model.dart
diff --git a/utils/pub/yaml/model.dart b/utils/pub/yaml/model.dart
index f709536fa79d0bfc2e536bbc2ee3efbdd66f3345..2ba1416b63145e8cd5936528d286a6858334a4e0 100644
--- a/utils/pub/yaml/model.dart
+++ b/utils/pub/yaml/model.dart
@@ -50,7 +50,7 @@ class _Tag {
 }
 
 /** The abstract class for YAML nodes. */
-class _Node {
+abstract class _Node {
   /** Every YAML node has a tag that describes its type. */
   _Tag tag;
 
@@ -66,7 +66,7 @@ class _Node {
 
   int get hashCode => _hashCode([tag, anchor]);
 
-  abstract visit(_Visitor v);
+  visit(_Visitor v);
 }
 
 /** A sequence node represents an ordered list of nodes. */