WeakSet for IE11 and engines with a WeakMap
npm install @ungap/weakset   !WebReflection status
A polyfill for IE11 or browsers that have a WeakMap, but not a WeakSet.
* CDN via https://unpkg.com/@ungap/weakset
* ESM via import WeakSet from '@ungap/weakset'
* CJS via const WeakSet = require('@ungap/weakset')
Compatible down to IE9, works well with ES5 shim upfront in IE8 (and maybe lower too).