Use dart.io condition to fix the import for mobile flutter apps

Author: HiDaN4

lib/logger.dart

@@ -17,8 +17,7 @@ export 'src/printers/simple_printer.dart';
 export 'src/printers/hybrid_printer.dart';
 export 'src/printers/prefix_printer.dart';
 
-export 'src/log_output.dart'
-    if (dart.library.io) 'src/outputs/file_output.dart';
+export 'src/log_output.dart' if (dart.io) 'src/outputs/file_output.dart';
 
 export 'src/log_filter.dart';
 export 'src/log_output.dart';