code: function _doSplit(orientation) {
          var firstPane, newPane;
  
          if (orientation === _orientation) {
              return;
          }
  
          firstPane = _panes[FIRST_PANE];
          Resizer.removeSizable(firstPane.$el);
  
          if (_orientation) {
              _$el.removeClass("split-" + _orientation.toLowerCase());
          }
          _$el.addClass("split-" + orientation.toLowerCase());
  
          _orientation = orientation;
          newPane = _createPaneIfNecessary(SECOND_PANE);
          _makeFirstPaneResizable();
  
          // reset the layout to 50/50 split
          // if we changed orientation then
          //  the percentages are reset as well
          _initialLayout();
  
          exports.trigger("paneLayoutChange", _orientation);
  
          // if new pane was created, and original pane is not empty, make new pane the active pane
          if (newPane && getCurrentlyViewedFile(firstPane.id)) {
              setActivePaneId(newPane.id);
          }
      }
docs: Creates a split for the specified orientation
  @private
  @param {!string} orientation (VERTICAL|HORIZONTAL)