add(aStr, aAllowDuplicates) {
    const isDuplicate = this.has(aStr);
    const idx = this._array.length;
    if (!isDuplicate || aAllowDuplicates) {
      this._array.push(aStr);
    }
    if (!isDuplicate) {
      this._set.set(aStr, idx);
    }
  }
Add the given string to this set.

@param String aStr