The builtin module test/mock_loader was added in v22.3.0. However, it wasn't added to schemelessBlockList, meaning it's currently importable as "test/mock_loader", and appears in module.builtinModules.

Could be as simple as adding the new module to the list, although I wonder if realm.js should implement some kind of matching behaviour so that if a module foo is in schemelessBlockList, then any foo/bar submodules are automatically excluded as well.