See https://github.com/avdg/UglifyJS2/blob/de619ae5a62472a76b0bfaa23d8e0b3068472196/lib/propmangle.js#L73

It makes uglify forget about js built-ins after the option is set to an other value. Maybe a separate option to merge the defaults may be a better option.