as(unit) {
    return this.isValid ? this.shiftTo(unit).get(unit) : NaN;
  }
Return the length of the duration in the specified unit.
@param {string} unit - a unit such as 'minutes' or 'days'
@example Duration.fromObject({years: 1}).as('days') //=> 365
@example Duration.fromObject({years: 1}).as('months') //=> 12
@example Duration.fromObject({hours: 60}).as('days') //=> 2.5
@return {number}