See https://github.com/web-platform-tests/wpt/pull/18722#issuecomment-526808403 I'm pretty sure this is a regression in webidl2.js, but I haven't minified the IDL snippet or bisected.
See web-platform-tests/wpt#18722 (comment)
I'm pretty sure this is a regression in webidl2.js, but I haven't minified the IDL snippet or bisected.