anchor.diff:  /// Helper class to launch a [dds.DartDevelopmentService]. Allows for us to
   /// mock out this functionality for testing purposes.
   class DartDevelopmentService {
  -  dds.DartDevelopmentService _ddsInstance;
  +  dds.DartDevelopmentService? _ddsInstance;
   
  -  Uri get uri => _ddsInstance?.uri ?? _existingDdsUri;
  -  Uri _existingDdsUri;
  +  Uri? get uri => _ddsInstance?.uri ?? _existingDdsUri;
  +  Uri? _existingDdsUri;
   
     Future<void> get done => _completer.future;
     final Completer<void> _completer = Completer<void>();
  
positive.code:           enableAuthCodes: !disableServiceAuthCodes,
            ipv6: ipv6,
          );
        unawaited(_ddsInstance.done.whenComplete(() {
          if (!_completer.isCompleted) {
            _completer.complete();
          }
        }));
        logger.printTrace('DDS is listening at ${_ddsInstance.uri}.');
      } on dds.DartDevelopmentServiceException catch (e) {
        logger.printTrace('Warning: Failed to start DDS: ${e.message}');
        if (e.errorCode == dds.DartDevelopmentServiceException.existingDdsInstanceError) {
  
negative.code:     int hostPort,
      bool ipv6,
      bool disableServiceAuthCodes, {
      required Logger logger,
    }) async {
      final Uri ddsUri = Uri(
        scheme: 'http',
  