anchor.diff:    /// Entries indicating the beginning of each span.
     final List<TargetLineEntry> lines;
   
  +  /// Url of the target file.
  +  String targetUrl;
  +
     /// Source root appended to the start of all entries in [urls].
  -  String sourceRoot = null;
  -
  -  SingleMapping._internal(this.targetUrl, this.urls, this.names, this.lines);
  +  String sourceRoot;
  +
  +  SingleMapping._(this.targetUrl, this.urls, this.names, this.lines);
   
     factory SingleMapping.fromEntries(
         Iterable<builder.Entry> entries, [String fileUrl]) {
  
positive.code: 
  /// A map containing direct source mappings.
  class SingleMapping extends Mapping {
    /// Url of the target file.
    final String targetUrl;
  
    /// Source urls used in the mapping, indexed by id.
    final List<String> urls;
  
  
negative.code:             srcNameId));
        }
      }
      return new SingleMapping._(
          fileUrl, urls.keys.toList(), names.keys.toList(), lines);
    }
  
  