code: function _forEachPaneOrPanes(paneId, callback) {
          if (paneId === ALL_PANES) {
              _.forEach(_panes, callback);
          } else {
              callback(_getPane(paneId));
          }
      }
docs: Iterates over the pane or ALL_PANES and calls the callback function for each.
  @param {!string} paneId - id of the pane in which to adjust the scroll state, ALL_PANES or ACTIVE_PANE
  @param {!function(!pane:Pane):boolean} callback - function to callback on to perform work.
  The callback will receive a Pane and should return false to stop iterating.
  @private