I'm sending a bug report

When i declare a class inside an IIFE, but class is not used, it isn't stripped by UglifyJS, because it is considered as a side effect.

var V6Engine = (function () {
    function V6Engine() {
    }
    V6Engine.prototype.toString = function () {
        return 'V6';
    };
    return V6Engine;
}());

The warning i get:

WARN: Side effects in initialization of unused variable V6Engine [./dist/car.bundle.js:74,4]

I suspect it is because it assigns values to the prototype.
The biggest pain point with it that it blocks tree-shaking of exported classes generated by Typescript.

Can it be fixed or is their a class declaration that is not considered a side effect?
Here is an example repository where i use UglifyJS through Webpack with Typescript.