anchor.diff:  
   const int noOffset = -1;
   
  -abstract class RoundTripFailure {
  +abstract class TextSerializationVerificationFailure {
     /// [Uri] of the file containing the expression that produced an error during
     /// the round trip.
     final Uri uri;
  
positive.code:         writeNode(deserialized, dartTypeSerializer, uri, offset);
  
      if (initial != serialized) {
        failures
            .add(new RoundTripMismatchFailure(initial, serialized, uri, offset));
      }
    }
  
  
negative.code:     try {
        result = serializer.readFrom(stream);
      } catch (exception) {
        failures.add(
            new TextDeserializationFailure(exception.toString(), uri, offset));
      }
      if (stream.moveNext()) {
        failures.add(new TextDeserializationFailure(
            "unexpected trailing text", uri, offset));
      }
      return result;
  