build_node_compilers not working on null safe library

[agreaves]

We're running build_node_compilers on a library that has been completely migrated to null safety, however we're getting the following types of errors:

packages/package_config/src/package_config_json.dart:311:28:
Error: Null safety features are disabled for this library.
  if (object is List<Object?>) {

dart2js is not able to convert libraries with any null safety features and we're not sure why at the moment. This is new as of Dart 2.14.0.