Error handler slot; events carry a message when available.
Message handler slot; events carry the payload in data.
Registers an event listener. Only used for the runtime-specific
worker-ended event named by WebWorkerHostOptions.exitEventName,
which is not part of the web Worker standard and therefore has no
handler slot.
Posts a structured-clonable message to the worker.
Terminates the worker.
Minimal shape of a web
Workerinstance as used by this host.