anchor.diff:            description = source.parseDescription(filePath, description,
                 fromLockFile: true);
           } on FormatException catch (ex) {
  -          throw new SpanFormatException(ex.message, spec.nodes['source'].span);
  +          throw new SourceSpanFormatException(ex.message,
  +              spec.nodes['source'].span);
           }
   
           var id = new PackageId(name, sourceName, version, description);
  
positive.code:   /// If [condition] is `false` throws a format error with [message] for [node].
    static void _validate(bool condition, String message, YamlNode node) {
      if (condition) return;
      throw new SpanFormatException(message, node.span);
    }
  
    /// Returns the serialized YAML text of the lock file.
  
negative.code: library pub.lock_file;
  
  import 'package:path/path.dart' as p;
  import 'package:source_span/source_span.dart';
  import 'package:yaml/yaml.dart';
  
  import 'io.dart';
  