Fix analyzer errors/warnings for Pub.

utils/pub/yaml/visitor.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;
+
 /** 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.keys) {
       out[key.visit(this)] = map.content[key].visit(this);
     }
     return out;
   }
 }