code: function ensureMap(set) {
              if (!set['[[SetData]]']) {
                var m = set['[[SetData]]'] = new collectionShims.Map();
                Object.keys(set._storage).forEach(function(k) {
                  // fast check for leading '$'
                  if (k.charCodeAt(0) === 36) {
                    k = k.substring(1);
                  } else {
                    k = +k;
                  }
                  m.set(k, k);
                });
                set._storage = null; // free old backing storage
              }
            }
docs: Switch from the object backing storage to a full Map.