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