Expose LogOutput class unconditionally

Author: Christophe Oosterlynck

lib/logger.dart

@@ -5,6 +5,8 @@ export 'src/ansi_color.dart';
 export 'src/filters/development_filter.dart';
 export 'src/filters/production_filter.dart';
 export 'src/log_filter.dart';
+// [LogOutput] class needs to be exposed unconditionally
+export 'src/log_output.dart';
 export 'src/log_output.dart'
     if (dart.library.io) 'src/outputs/file_output.dart';
 export 'src/log_printer.dart';