[frontend_server] Export to_string_transformer from frontend_server.dart

Change-Id: I0a3dae9c6d9357b71539ceacc16d0cf4f544ba69
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/179500
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/frontend_server/lib/frontend_server.dart

@@ -36,6 +36,8 @@ import 'package:vm/kernel_front_end.dart';
 import 'src/javascript_bundle.dart';
 import 'src/strong_components.dart';
 
+export 'src/to_string_transformer.dart';
+
 ArgParser argParser = ArgParser(allowTrailingOptions: true)
   ..addFlag('train',
       help: 'Run through sample command line to produce snapshot',