src/common/fast-path.js/getName
FastPath.prototype.getName = function getName() {
  const s = this.stack;
  const len = s.length;
  if (len > 1) {
    return s[len - 2];
  }
  // Since the name is always a string, null is a safe sentinel value to
  // return if we do not know the name of the (root) value.
  /* istanbul ignore next */
  return null;
};