anchor.diff:              srcNameId));
         }
       }
  -    return new SingleMapping._internal(
  +    return new SingleMapping._(
           fileUrl, urls.keys.toList(), names.keys.toList(), lines);
     }
   
  
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: 
  /// A map containing direct source mappings.
  class SingleMapping extends Mapping {
    /// Source urls used in the mapping, indexed by id.
    final List<String> urls;
  
  