In practice not many movies will care about this, because the `AS3` namespace is open by default. You could opt-out of that, and I suppose that was there for using existing ES3 code in AS3 projects. ES4 would have had a similar ES4 namespace, which "JavaScript 2.0" code would need to opt into. Of course, ES4/JS2 never happened, so we just have this weird historical quirk here.
Private names now return `false`, and we run any names through trait lookups. This also means any namespace resolution can fail now, in case we need to throw a `VerifyError`.
All constant pools in an ABC file are actually numbered starting from one; there's an implicit 0 entry not stored in the file that the runtime is expected to retrieve when pulling constants from the pool.
The AVM2/ABC spec only mentions this in passing.