function prepareBoxModel(tweenProp, value) {
    var boxValue = trueDimension(value); var
      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 value
@returns {number} the property tween object