Runtime APIs: MessageChannel

[phase]

The MessageChannel API is available on node but not CF Workers. This is causing an issue during the deployment of sites using React 19 + Astro + Workers, see facebook/react#31827 and withastro/astro#12824. The usage of this API may be a bug for this use case (meaning the solution there may be to stop using MessageChannel), but workerd may want to support it anyway.

I made a repository with steps to reproduce the crash mentioned in those issues here: https://github.com/phase/test-astro-react19-cf-workers

[jasnell]

Relabeling this as a feature request. The fact that we currently do not support MessageChannel is currently intentional as there aren't really any usable use cases for it in the runtime currently. It is something we can revisit but it likely won't be soon. To help with that, it would be helpful to have a better idea of how MessageChannel is being used in this particular case.