Fix a variable name typo in yaml.

pkg/yaml/lib/src/parser.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.parser;
 
 import 'dart:collection';
 
 import 'package:source_maps/source_maps.dart';
 import 'package:string_scanner/string_scanner.dart';
 
 import 'equality.dart';
 import 'model.dart';
 import 'utils.dart';
-import 'yaml_exception.dart';
 
 /// Translates a string of characters into a YAML serialization tree.
 ///
 /// This parser is designed to closely follow the spec. All productions in the
 /// spec are numbered, and the corresponding methods in the parser have the same
 /// numbers. This is certainly not the most efficient way of parsing YAML, but
 /// it is the easiest to write and read in the context of the spec.
 ///
 /// Methods corresponding to productions are also named as in the spec,
 /// translating the name of the method (although not the annotation characters)
@@ skipping 1866 matching lines @@
     for (var pair in _contents.reversed) {
       if (pair.first.contains(pos)) return pair.last;
     }
     return null;
   }
 
   /// Associates [value] with [range].
   operator[]=(_Range range, E value) =>
     _contents.add(new Pair<_Range, E>(range, value));
 }