code: function combineErrors(errPrev, errNew) {
    if (!errPrev) {
      return errNew;
    }
  
    let errCombined;
    if (errPrev instanceof AggregateError) {
      errCombined = errPrev;
    } else {
      errCombined = new AggregateError();
      errCombined.push(errPrev);
    }
  
    errCombined.push(errNew);
    return errCombined;
  }
docs: Combines one or more errors into a single error.
  
  @param {AggregateError|Error} errPrev Previous errors, if any.
  @param {!Error} errNew New error.
  @return {!AggregateError|!Error} Error which represents all errors that have
  occurred.  If only one error has occurred, it will be returned.  Otherwise
  an {@link AggregateError} including all previous errors will be returned.
  @private