Object.assign does not throw when trying to extend sealed object

[StyMaar]

(function(){
    "use strict";
    let toto = {
        a:1,
        b:2,
        c:3
    }

    let conf = {
        a:undefined,
        b:undefined,
    }

    Object.seal(conf);

    Object.assign(conf,toto);
    console.log(conf);
})();

In Firefox the given sample throws because Object.assign tries to add a c property to the sealed conf object.
With Babel-node it works and assign only the attribute a and b.

I think that the Firefox approach is the expected one.

[zloirock]

Fixed in 7bf885b