babel-minify breaks polyfills added to global built-in objects

Suppose that I try to add a new method to the global `Math` object as follows:

(function () {
Math["addOne"] = function(x) {return x + 1};
console.log(Math.addOne(1), Math.addOne(2));
}());

Then minifying that with the `babel-minify` cli application, I get the following output (pretty printed):

(function() {
var a = Math.addOne;
Math.addOne = function(a) {
return a + 1
},
console.log(a(1), a(2))
}
)();

As you can see, an alias to the method is created, but that happens before the method was defined, so the alias will be invalid and the code will not work.

If change `Math["addOne"] = ...` to `Math.addOne = ...` in the definition of the method, then everything works fine. The problem is that in my actual code, I cannot control how the method is defined, because the method is a polyfill for `Math.sign` which is provided by `core-js`, which uses `Object.defineProperty` to define it.