function setActivePaneId(newPaneId) {
        if (!_isSpecialPaneId(newPaneId) && newPaneId !== _activePaneId) {
            var oldPaneId = _activePaneId,
                oldPane = _getPane(ACTIVE_PANE),
                newPane = _getPane(newPaneId);

            if (!newPane) {
                throw new Error("invalid pane id: " + newPaneId);
            }

            _activePaneId = newPaneId;

            exports.trigger("activePaneChange", newPaneId, oldPaneId);
            exports.trigger("currentFileChange", _getPane(ACTIVE_PANE).getCurrentlyViewedFile(),
                                                            newPaneId,
                                                            oldPane.getCurrentlyViewedFile(),
                                                            oldPaneId);

            _makePaneMostRecent(_activePaneId);
            focusActivePane();
        }
    }
Switch active pane to the specified pane id (or ACTIVE_PANE/ALL_PANES, in which case this
call does nothing).
@param {!string} paneId - the id of the pane to activate