function getMainCarrier() {
    var carrier = getGlobalObject();
    carrier.__SENTRY__ = carrier.__SENTRY__ || {
        extensions: {},
        hub: undefined,
    };
    return carrier;
}
Returns the global shim registry.

FIXME: This function is problematic, because despite always returning a valid Carrier,
it has an optional `__SENTRY__` property, which then in turn requires us to always perform an unnecessary check
at the call-site. We always access the carrier through this function, so we can guarantee that `__SENTRY__` is there.