They don't seem to expose the List to be accessible from the instanced Object. This is from the EcmaScript Draft:

23.2.4 Properties of Set Instances

Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.

[[SetData]] is the list of values the Set is holding.

A possible solution (and a somewhat expensive one) is to grab an iterator and then call next() for the first value:

var x = new Set();

x.add(1);

x.add({ a: 2 });

//get iterator:

var it = x.values();

//get first entry:

var first = it.next();

//get value out of the iterator entry:

var value = first.value;

console.log(value); //1

Worth mentioning too:

Set.prototype.values === Set.prototype.keys