When generating global mixins, only define regular members from the
[Global] interface itself on the global object. Inherited members remain
available through the prototype chain.
This was found due to a timeout on:
https://wpt.live/html/dom/idlharness.any.worker.html
Installing inherited members created fresh own functions on worker
globals, so self.importScripts differed from
WorkerGlobalScope.prototype.importScripts. That confused idlharness into
taking its null-this check path for importScripts, which then attempted
to import "null" and timed out.
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