Webdriver servo async JS script breaks some wdspec tests

[jdm]

In particular, tests/wpt/web-platform-tests/webdriver/tests/add_cookie/user_prompts.py uses:

        session.execute_async_script("""
            let dialog_type = arguments[0];
            let text = arguments[1];

            setTimeout(function() {
              if (dialog_type == 'prompt') {
                window.dialog_return_value = window[dialog_type](text, '');
              } else {
                window.dialog_return_value = window[dialog_type](text);
              }
            }, 0);
            """, args=(dialog_type, text))

which ends up yielding this error:

 0:06.21 pid:15764 [2020-06-22T14:23:42Z ERROR script::dom::bindings::error] Error at :10:65 window[dialog_type] is not a function

because dialog_type is a reference to the webdriverCallback method on Window instead of the a string like 'alert' or 'prompt'.