anchor.diff:          writeNode(deserialized, dartTypeSerializer, uri, offset);
   
       if (initial != serialized) {
  -      failures
  -          .add(new RoundTripMismatchFailure(initial, serialized, uri, offset));
  +      failures.add(new TextRoundTripFailure(initial, serialized, uri, offset));
       }
     }
   
  
positive.code: 
  const int noOffset = -1;
  
  abstract class RoundTripFailure {
    /// [Uri] of the file containing the expression that produced an error during
    /// the round trip.
    final Uri uri;
  
negative.code:         writeNode(deserialized, expressionSerializer, uri, offset);
  
      if (initial != serialized) {
        failures.add(new TextRoundTripFailure(initial, serialized, uri, offset));
      }
    }
  
  