[dart2js] Fix broken dart2js_batch_test.

Fixes: https://github.com/dart-lang/sdk/issues/55391
Change-Id: If4fdfb45348811d0f563baa30b68173b8ce0e444
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/363460
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/compiler/test/end_to_end/launch_helper.dart

@@ -23,7 +23,6 @@ List<String> dart2JsCommand(List<String> args) {
     basePath = parentPath;
   }
   final command = <String>[];
-  command.add('--no-sound-null-safety');
   if (Platform.packageConfig != null) {
     command.add('--packages=${Platform.packageConfig}');
   }