Closes https://github.com/denoland/deno/issues/21828.
This API is a huge footgun. And given that "Deno.serveHttp" is a
deprecated API that is discouraged to use (use "Deno.serve()"
instead); it makes no sense to keep this API around.
This is a step towards fully migrating to Hyper 1.
This fixes #21434 for `BroadcastChannel` and `WebSocketStream`.
`--unstable` still enable both, but granular unstable flags now also
work:
* `--unstable-net` now enables `WebSocketStream`.
* `--unstable-broadcast-channel` now enables `BroadcastChannel`.
* Additionally, there are now tests for all granular unstable flags.
Since `unsafe-proto` already had tests, so I didn't add any for this
one.
It also introduces a map to keep track of granular unstable ids without
having to sync multiple places.
