Implement the constructor for EventSource

[KiChjang]

Partial #8925.

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @KiChjang: components/script/dom/eventsource.rs, components/script/dom/request.rs, components/script/fetch.rs

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!
• These commits modify script code, but no tests are modified. Please consider adding a test!

[KiChjang]

@bors-servo try

[bors-servo]

⌛ Trying commit 65c6dd4 with merge 345a9d5...

[bors-servo]

☀️ Test successful - arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css, mac-rel-wpt1, mac-rel-wpt2, windows-dev

[Ms2ger]

It's not clear to me why we'd want to create a js promise here at all.

[KiChjang]

How else would you fetch the request?

[Ms2ger]

With all the usual infra, like you did for the script element.

[KiChjang]

An EventSource needs to have an associated Request object. I recall @jdm saying that we should store a dom::Request instead of a net_traits::Request, and by doing so, the straightforward way to fetch a dom::Request would be via dom::Fetch. I could instead make a function that transforms this dom::Request into a RequestInit, and then properly fetch it using the document_loader stuff, but if so, I think we're better off storing a RequestInit instead.

[KiChjang]

Not sure if there's any value in merging just the EventSource constructor here, since it doesn't make any failing tests pass.

[Ms2ger]

Agreed.