Utilities for Various Builtins #2
The Utility class should keep track of its own activation state, otherwise calling .activate() multiple times on the same instance (without calling .deactivate() in between) might cause some problems; also, in the current code, calling .addUtil() after .activate() leaves the instance in a limbo state, where the method name is added to this.utils but isn't actually available on the aided class's prototype, even though the instance is activated. I can imagine this leading to some frustrating debugging on a more complex project. I'd suggest that calling .addUtil() on an activated Utility instance should either immediately activate the added function, or throw an Error (this sounds more consistent with your approach).

(also, as a side note, I wouldn't call this syntactic sugar. Sugar is just simpler way of writing something. e.g. { ...obj } is sugar for Object.assign({}, obj); it doesn't actually add new functionality, which your code certainly does)