[whatwg/dom] Use undefined union for Window.event (#935)

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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/935

-- Commit Summary --

  * Use undefined union for Window.event

-- File Changes --

    M dom.bs (2)

-- Patch Links --

https://github.com/whatwg/dom/pull/935.patch
https://github.com/whatwg/dom/pull/935.diff

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