anchor.diff:  library pub.lock_file;
   
   import 'package:path/path.dart' as p;
  -import 'package:source_maps/source_maps.dart';
  +import 'package:source_span/source_span.dart';
   import 'package:yaml/yaml.dart';
   
   import 'io.dart';
  
positive.code: 
      if (contents.trim() == '') return new LockFile.empty();
  
      var sourceName;
      if (filePath != null) sourceName = p.toUri(filePath).toString();
      var parsed = loadYamlNode(contents, sourceName: sourceName);
  
      _validate(parsed is Map, 'The lockfile must be a YAML mapping.', parsed);
  
  
negative.code:           description = source.parseDescription(filePath, description,
                fromLockFile: true);
          } on FormatException catch (ex) {
            throw new SourceSpanFormatException(ex.message,
                spec.nodes['source'].span);
          }
  
          var id = new PackageId(name, sourceName, version, description);
  