_stringArrayToSet(strArray, errMsg) {
    if (!strArray) return {};

    errMsg =
      errMsg || 'Cannot make set from type other than Array of string elements';

    if (!Array.isArray(strArray)) {
      throw new Error(errMsg);
    }

    return strArray.reduce((set, el) => {
      if (typeof el !== 'string') {
        throw new Error(errMsg);
      }
      set[el] = true;

      return set;
    }, {});
  }
Returns a Set-like object with strArray's elements as keys (each with the
value true).
@param {Array} strArray - Array of Set-elements as strings.
@param {?string} [errMsg] - Custom error message thrown on invalid input.
@returns {Object} - TODO: add return description.
@private