Spec: https://html.spec.whatwg.org/multipage/web-sockets.html#dom-websocket

The "Establish a WebSocket connection" step is inside "in parallel", which means that it shouldn't run synchronously before the constructor algorithm returns. In the spec, port blocking happens in https://fetch.spec.whatwg.org/#port-blocking, which is within "Establish a WebSocket connection".

Recently updated tests are consistently failing because of this:
https://staging.wpt.fyi/results/websockets/Create-blocked-port.any.html?sha=eea0b54014&label=experimental

They are also failing in other browsers, so if this change doesn't seem good, please file an issue on https://github.com/whatwg/html/issues/new to change it.