The regular random number generator in Flash is automatically seeded and you can't specify a seed. This article, though, talks about using BitmapData.noise() which takes a seed as a parameter, and normally when I see people discussing the use of that API for general purpose random number generation it is precisely because it allows control over seeding.
A diligent coder can of course implement their own PRNG method, but that is not an excuse for providing a half-broken feature.