Comment 2 for bug 1683445

So as a wokraround:

sudo vim /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

sudo update-grub

reboot

This option adds the dell_backlight:
ubuntu@ubuntu-Latitude-E6430:~$ ls /sys/class/backlight
dell_backlight intel_backlight nv_backlight

Note that sending events to the intel_backlight like so:
root@ubuntu-Latitude-E6430:~# echo 1000 > /sys/class/backlight/intel_backlight/brightness
works too.

More info regarding backlight troubleshooting:
http://hansdegoede.livejournal.com/13889.html

I guess we need a kernel patch/quirk.