Register service extensions when the Service stream is listened to

[elliette]

Work towards flutter/devtools#7313

Copy the VM service behavior of registering service extensions when a client subscribes to the Service stream: https://github.com/dart-lang/sdk/blob/main/sdk/lib/vmservice/vmservice.dart#L588

Not a complete fix, because DWDS' streamListen implementation is not being called in response to DevTools streamListen here: https://github.com/flutter/devtools/blob/20c66f853252bb7199c9bc2dafa17f3f28dee0d0/packages/devtools_app_shared/lib/src/service/service_manager.dart#L343

[bkonyi]

To be clear, the VM service's implementation doesn't actually register the extensions when the Service stream is listened to, it just posts ServiceRegistered events for each extension so the client knows which services are currently registered. In theory, the client doesn't have to listen to the Service stream and can get the list of service extensions from the Isolate object and invoke them that way.

Is there no way for DWDS to register these service extensions at startup?

[bkonyi]

Ah, nevermind, we register them against each individual client... let's meet and discuss when you have a chance, I think I might be missing some context.

[elliette]

More work is required here, closing this.