function _resolvePaneId(paneId) {
        if (!paneId || paneId === ACTIVE_PANE) {
            return getActivePaneId();
        }
        return paneId;
    }
Resolve paneId to actual pane.
@param {?string} paneId - id of the desired pane. May be symbolic or null (to indicate current pane)
@return {string} id of the pane in which to open the document