code: function _getPane(paneId) {
          paneId = _resolvePaneId(paneId);
  
          if (_panes[paneId]) {
              return _panes[paneId];
          }
  
          return null;
      }
docs: Retrieves the Pane object for the given paneId
  @param {!string} paneId - id of the pane to retrieve
  @return {?Pane} the Pane object or null if a pane object doesn't exist for the pane
  @private