function _getDisplayKey(key) {
        var displayKey = "",
            match = key ? key.match(/(Up|Down|Left|Right|\-)$/i) : null;
        if (match && !/Page(Up|Down)/.test(key)) {
            displayKey = key.substr(0, match.index) + _displayKeyMap[match[0].toLowerCase()];
        }
        return displayKey;
    }
@private

Gets the corresponding unicode symbol of an arrow key for display in the menu.
@param {string} key The non-modifier key used in the shortcut. It does not need to be normalized.
@return {string} An empty string if key is not one of those we want to show with the unicode symbol.
                 Otherwise, the corresponding unicode symbol is returned.