negativeFormat() {
    return this.numberFormatFormatter().format({
      alwaysShowsDecimalSeparator: this.alwaysShowsDecimalSeparator(),
      groupingSize: this.groupingSize(),
      maximumFractionDigits: this.maximumFractionDigits(),
      minimumFractionDigits: this.minimumFractionDigits(),
      minimumIntegerDigits: this.minimumIntegerDigits(),
      prefix: this._get('negativePrefix'),
      suffix: this._get('negativeSuffix'),
      usesGroupingSeparator: this.usesGroupingSeparator()
    });
  }
Gets the negative number format string for the current settings. For
example, changing `minimumFractionDigits` from 0 to 3 would change this
value from "-#" to "-#.000".

@return {string}