anchor.diff:  
   /// 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;
   
  
positive.code:   /// Entries indicating the beginning of each span.
    final List<TargetLineEntry> lines;
  
    /// Source root appended to the start of all entries in [urls].
    String sourceRoot = null;
  
    SingleMapping._internal(this.targetUrl, this.urls, this.names, this.lines);
  
    factory SingleMapping.fromEntries(
        Iterable<builder.Entry> entries, [String fileUrl]) {
  
negative.code:             srcNameId));
        }
      }
      return new SingleMapping._(
          fileUrl, urls.keys.toList(), names.keys.toList(), lines);
    }
  
  