Symbol values should be accepted as a WeakMap key

const map = new WeakMap();
const sym = Symbol();
map.set(sym, 42);

This code is supposed to work in ES6, but it raises a TypeError instead.

https://tc39.es/ecma262/multipage/keyed-collections.html#sec-weakmap-objects

"WeakMaps are collections of key/value pairs where the keys are objects and/or symbols"