Fix analyzer errors/warnings for Pub.

utils/pub/yaml/composer.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 YAML document (what the spec calls the "serialization tree")
  * and resolves aliases, resolves tags, and parses scalars to produce the
  * "representation graph".
  */
 class _Composer extends _Visitor {
   /** The root node of the serialization tree. */
   _Node root;
 
   /**
@@ skipping 157 matching lines @@
           value: Math.parseDouble("NaN"));
     }
 
     return null;
   }
 
   /** Parses a string scalar. */
   _ScalarNode parseString(String content) =>
     new _ScalarNode(_Tag.yaml("str"), value: content);
 }