fix(ext/flash): Fix request object returning uri property correctly after websocket upgrade

[andrewnester]

Currently this is what's happening using the example from #18490

1. When websocket connection established, the following function executes and adds the request with token 0 in context https://github.com/denoland/deno/blame/main/ext/flash/lib.rs#L1377
2. After that websocket is upgraded (https://github.com/denoland/deno/blob/main/ext/flash/01_http.js#L425) which removes the request with token 0 from the context https://github.com/denoland/deno/blame/main/ext/flash/lib.rs#L1456
3. open event is emitted after websocket upgraded is executed https://github.com/denoland/deno/blob/main/ext/flash/01_http.js#L435, therefore on open callbacks are executed later
4. If we request request.uri option inside open callback, it is lazy loaded via this function https://github.com/denoland/deno/blob/main/ext/flash/01_http.js#L574 which requires request with token 0 in the server context which was removed on step 2. Thus panic.

This PR fixes this issue by preloading path before executing flash request.

Closes #18490

[bartlomieju]

Thanks for the PR @andrewnester, we're currently in the process for migrating away from using the "flash" server (with #18511 being the prerequisite), so I'm not sure if we'll land this PR.

[bartlomieju]

@andrewnester thank you so much for the PR, but this issue got fixed in #18568. I'm going to close it without a merge. I really appreciate the contribution, sorry I didn't let you know that we're working on it in the issue.