It seems that isProtoString will always return false?

https://github.com/mozilla/source-map/blob/58819f09018d56ef84dc41ba9c93f554e0645169/lib/util.js#L66

function isProtoString(s) {
  if (!s) {
    return false;
  }

  const length = s.length;

  if (length < 9 /* "__proto__".length */) {
    return false;
  }

  /* eslint-disable no-multi-spaces */
  if (
    s.charCodeAt(length - 1) !== 95 /* '_' */ ||
    s.charCodeAt(length - 2) !== 95 /* '_' */ ||
    s.charCodeAt(length - 3) !== 111 /* 'o' */ ||
    s.charCodeAt(length - 4) !== 116 /* 't' */ ||
    s.charCodeAt(length - 5) !== 111 /* 'o' */ ||
    s.charCodeAt(length - 6) !== 114 /* 'r' */ ||
    s.charCodeAt(length - 7) !== 112 /* 'p' */ ||
    s.charCodeAt(length - 8) !== 95 /* '_' */ ||
    s.charCodeAt(length - 9) !== 95 /* '_' */
  ) {
    return false;
  }
  /* eslint-enable no-multi-spaces */

  for (let i = length - 10; i >= 0; i--) {
    if (s.charCodeAt(i) !== 36 /* '$' */) {
      return false;
    }
  }

  return true;
}

Start in lib/util.js at isProtoString, using the linked implementation and its callers to determine which strings the function is intended to recognize. Reproduce the reported behavior with representative inputs and inspect existing tests or test commands; done means the predicate returns the intended result and has regression coverage.