Get checked state of checkbox

Hello!

I'm building a plugin for CKEditor and have added a checkbox element to a dialog:

CKEDITOR.dialog.add('myplugin', function(editor)
{
    return {
        title: 'My Plugin',
        elements: [
            {
                type: 'checkbox',
                label: 'click me',
                onClick: function(event)
                {
                    // triggered when checkbox is clicked
                    var checkbox = event.sender;
                    console.log('clicked!', checkbox.getValue());
                }
            }
        ]
    };
});

How can I determine the "checked" state of the checkbox?

Looking over the API docs, there doesn't seem to be an "isChecked()" method, and CKEDITOR.dom.element#getValue() always returns "on" regardless of checked state. I also tried CKEDITOR.dom.element#getAttribute('checked') but that always returns null.

The only workaround I have found is to use vanilla javascript to test the "checked" property directly, but I'm afraid this might not be compatible with all browsers (ahem... IE).

Thanks!