negate() {
    if (!this.isValid) return this;
    const negated = {};
    for (const k of Object.keys(this.values)) {
      negated[k] = this.values[k] === 0 ? 0 : -this.values[k];
    }
    return clone(this, { values: negated }, true);
  }
Return the negative of this Duration.
@example Duration.fromObject({ hours: 1, seconds: 30 }).negate().toObject() //=> { hours: -1, seconds: -30 }
@return {Duration}