Use unsigned long long instead of unsigned long?

[domenic]

This is extremely unlikely to matter in practice, especially for something that as far as I can tell is only exposed as a return value, but see https://w3ctag.github.io/design-principles/#numeric-types for guidance around consolidating around more-native-to-JS types instead of adding wrapping behavior.

This might be important if we ever want to do the sort of things mentioned in whatwg/webidl#33 (comment)