Add sequenceId slot and fix effect promises

[nondebug]

Closes #200

The following tasks have been completed:

• Modified Web platform tests (link to pull request)

Implementation commitment:

• WebKit
• Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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Preview | Diff

[marcoscaceres]

@gterzian, do you want to check this also?

[marcoscaceres]

Will need to check if the original issue is observable in any browser or if there was code already preventing this. Might be good if we can come up with a test to confirm (seems it might be possible to do so).

[marcoscaceres]

@nondebug I'm still not sure what happens if the identifiers don't match... feels like there should be some kind of assert or something somewhere to assuring nothing is getting out of sync.

[gterzian]

LGTM, with a few nits to consider.

[marcoscaceres]

Adding @gterzian's suggestions would be great. Can review again after those updates.

[nondebug]

@gterzian Thanks for the suggestions!

@marcoscaceres Please take another look

[marcoscaceres]

approving, but we should add a test so I can check WebKit.

[marcoscaceres]

Filed bug https://bugs.webkit.org/show_bug.cgi?id=277838