refactor magic_numbers.dart (#2165)

Co-authored-by: Lasse R.H. Nielsen <lrn@google.com>

Author: devoncarew

pkgs/mime/lib/mime.dart

@@ -10,6 +10,6 @@
 library;
 
 export 'src/extension.dart';
-export 'src/mime_multipart_transformer.dart';
 export 'src/mime_shared.dart';
 export 'src/mime_type.dart';
+export 'src/multipart_transformer.dart';