The only problem here is the

If argument has a [[RegExpMatcher]] internal slot, return true.

It's not possible for JavaScript alone to check or access internal slots (without native code being provided that does so). For the same reason, if you do someMap.set(key, value), it's impossible to retrieve either the key or the value from the Map unless you invoke one of the methods provided for such a thing, such as Map.prototype.entries or Map.prototype.keys, which invoke native code to look up the map keys/values in an internal slot.

So, that step with [[RegExpMatcher]] is impossible to replicate exactly.

That said, regular expressions are instances of RegExp, so 99% of the time, in sane code, a plain instanceof check will work.

console.log(/foo/ instanceof RegExp);
console.log(Object.getPrototypeOf(/foo/) === RegExp.prototype);

I don't know of any existing converter, but if your target is specifically ActionScript and not just any ECMA-262 implementation, the job could be easier than you expected. AS3 is powered by PCRE, same as PHP's preg_ functions, so it supports lookbehind, atomic groups and possessive quantifiers, same as Java. It also supports Java's dotall and extended modes in addition to JS's ignore-case and multiline. It supports the inline modifier syntax ((?imsx)) as well.

PCRE's Unicode support is better than Java's, but unfortunately, I don't think that's included in ActionScript. The Unicode functionality seems to be explicitly tied to the character encoding, which is UTF-8 in PCRE; I believe ActionScript uses UTF-16 in accordance with ECMA-262. Anyway, its Unicode support seems to be minimal, same as JavaScript's.