Flutterby™! : secure random password in JavaScript

How to create a secure random password with JavaScript. Probably stuff you know, about modulo bias and using decent sources of randomness and so forth (although at least one of my little one-off (non-JS) scripts that manages one of my personal sites has random number collisions far more often than I think it should for reasons I've never bothered to fix because it's just an annoyance, but...), but it's in one place.