function prepareBoxModel(tweenProp, value) {
  const boxValue = trueDimension(value);
  const offsetProp = tweenProp === 'height' ? 'offsetHeight' : 'offsetWidth';
  return boxValue.u === '%' ? (boxValue.v * this.element[offsetProp]) / 100 : boxValue.v;
}
Returns the property tween object.
@param {string} tweenProp the property name
@param {string} value the property name
@returns {number} the property tween object