Make YamlException inherit from SpanFormatException.

pkg/yaml/lib/src/model.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.
 
 /// This file contains the node classes for the internal representations of YAML
 /// documents. These nodes are used for both the serialization tree and the
 /// representation graph.
 library yaml.model;
 
 import 'package:source_maps/source_maps.dart';
@@ skipping 190 matching lines @@
           } else if (c >= 0x10000) {
             return "\\u${zeroPad(c.toRadixString(16).toUpperCase(), 8)}";
           } else {
             return new String.fromCharCodes([c]);
           }
         }
       });
       return '"${escapedValue.join()}"';
     }
 
-    throw new YamlException('Unknown scalar value: "$value".');
+    throw new YamlException('Unknown scalar value.', span);
   }
 
   String toString() => '$tag "$content"';
 
   /// Left-pads [str] with zeros so that it's at least [length] characters
   /// long.
   String zeroPad(String str, int length) {
     assert(length >= str.length);
     var prefix = new List.filled(length - str.length, '0');
     return '${prefix.join()}$str';
@@ skipping 28 matching lines @@
     var strContent = content.keys
         .map((k) => '${k}: ${content[k]}')
         .join(', ');
     return '$tag {$strContent}';
   }
 
   int get hashCode => super.hashCode ^ deepHashCode(content);
 
   visit(Visitor v) => v.visitMapping(this);
 }