Const is a pretty good example, the closest I can think to emulating it in es5 is something like:

Object.defineProperty(this, 'five', { enumerable: true, writable: false, value: 5, configurable: false });

If in global scope, 'five' will look like a const.

WeakMap and WeakSet have polyfills with minor caveats.

I guess you're defining language features as not "syntactic sugar" if they can't be absolutely 100% polyfilled.