[act] Wrap IsThisRendererActing in DEV check

[acdlite]

So that it doesn't leak into the production bundle. Follow-up to #16240.

[sizebot]

No significant bundle size changes to report.

Generated by 🚫 dangerJS

[acdlite]

@threepointone Just to confirm, this isn't supposed to affect prod, right? There are some tests that are failing (correction: one test) but I assume that was just an oversight in the last PR.

[acdlite]

I'm going to assume that this doesn't affect prod and we can follow up later if that's not the case. All the act stuff should be dev-only, or alternatively, in a separate testing build.

[threepointone]

We use this sigil not just for warnings now, but also for deciding when to flush, so it does actually affect prod?

[acdlite]

Is this the only case? We really should not be leaking act behavior into the main production bundles.

[acdlite]

What is the use case for running act tests in production?

[threepointone]

I think the discussion we had was people might be running tests with prod builds? iirc. Happy to not support it if so, or move to making testing builds.

[acdlite]

Since this only affects Concurrent and Sync mode I'll merge this and we can pick up discussion elsewhere.