anchor.diff:    // spew a stack trace on our users.
     if (exception is SpanException) {
       error(exception.toString(useColors: canUseSpecialChars));
  +  } else if (exception is SourceSpanException) {
  +    error(exception.toString(color: canUseSpecialChars));
     } else {
       error(getErrorMessage(exception));
     }
  
positive.code:     }
  
      // If the error came from a file, include the path.
      if (error is SpanException && error.span.sourceUrl != null) {
        errorJson["path"] = p.fromUri(error.span.sourceUrl);
      }
  
  
negative.code: 
  import 'package:path/path.dart' as p;
  import 'package:source_maps/source_maps.dart';
  import 'package:source_span/source_span.dart';
  import 'package:stack_trace/stack_trace.dart';
  
  import 'exceptions.dart';
  