The work in bug 1642952 is simplifying workflows, and making it so that we load the app provided engines from the configuration first, then loading other engines.

Once that is complete, we should re-evaluate if we really need to store _isBuiltin in the cache. I believe it is now only used as a flag to make it easier when loading from the cache.

If that's the case, then we may already have the information about if an engine is built-in or not from the already-added engines list.

We should investigate to see if this is viable or not, if it is, and can be done within the allocated points, then we should do it, otherwise split it out to another bug.

With the legacy search configuration removed, and having thought about this a bit, we have decided not to do this.

The flag is a useful optimisation that allows us to avoid having to check if each engine in the cache is an app-provided one (by doing a Map.has() lookup). The trade-off is storing the per-engine flag vs taking longer per engine to check. Although most users don't generally have lots of engines installed, we still think it is better to default to having the flag rather than not.