Fix a variable name typo in yaml.

pkg/yaml/lib/src/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.
 
 library yaml.composer;
 
-import 'package:source_maps/source_maps.dart';
-
 import 'model.dart';
 import 'visitor.dart';
 import 'yaml_exception.dart';
 
 /// 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.
   final Node _root;
@@ skipping 154 matching lines @@
       return new ScalarNode(Tag.yaml("float"), scalar.span, value: double.NAN);
     }
 
     return null;
   }
 
   /// Parses a string scalar.
   ScalarNode parseString(ScalarNode scalar) =>
     new ScalarNode(Tag.yaml("str"), scalar.span, value: scalar.content);
 }