Does the phrase "syntactic sugar" have any meaning? Name a new language feature that "it is just syntactic sugar" can't be applied to.
Or, for that matter, const. Though perhaps const is just syntatic sugar for not reassigning to the same darn name? ;)
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.