Fix analyzer errors/warnings for Pub.

utils/pub/yaml/constructor.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.
 
+part of yaml;
+
 /**
  * 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. */
@@ skipping 34 matching lines @@
     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;
   }
 }