function getOppositeVariation(variation) {
    if (variation === 'end') {
      return 'start';
    } else if (variation === 'start') {
      return 'end';
    }
    return variation;
  }
Get the opposite placement variation of the given one
@method
@memberof Popper.Utils
@argument {String} placement variation
@returns {String} flipped placement variation