Custom event args cannot include byte arrays or DotNetObject instances

[SteveSandersonMS]

This is a small bug introduced recently (by me, I think) due to an optimization. Update: No, after investigation, I realise this was never supported, because the event dispatch was not via JS interop (on Server and WebView).

Our updated logic for dispatching events now does its own JSON serialization (so it can do its own UTF8 encoding, so the .NET side doesn't have to convert UTF8->String->UTF8). The logic looks like const utf8 = textEncoder.encode(JSON.stringify(data));

This is inconsistent with how the serialization works for JS interop, in that JS interop uses its own arg replacer that deals with things like formatting DotNetObject or Uint8Array.

We do actually have an E2E test showing you can include special types in the event payload, but unluckily, it happens to be IJSObjectReference that happens to serialize into the right shape using JSON.stringify alone.

Fixing this

... should be very simple. We export some new function stringifyAsArgs or similar onto JS interop's DotNet, and call that instead of JSON.stringify.

Better still, we should bake in this "I want to send it as UTF8 bytes" pattern, since we're using it in multiple places. We could export a special type Utf8BytesParameter from DotNet that changes the serialization rule for that parameter.

Update: No, those suggestions are nonsense. The second one doesn't make sense because we're not sending the message via JS interop, so there's nothing to implement this. The first one is bad because it breaks the layering and means we wouldn't be able to guarantee that things like byte[] messages correctly appear just before corresponding JS interop calls. A bigger solution is needed.

[SteveSandersonMS]

Make sure the fix also covers #34339 (comment)

[SteveSandersonMS]

On further investigation, this does not actually appear to be a regression. It looks as if we've never supported byte arrays or DotNetObject instances in custom event args data, since we've always called JSON.stringify inside boot.*.ts on the event args object.