Although, I have not seen an in-built feature in phone to enable the flash but there is a useful app called PhoneTorch for the N9.
It is a very simple app and available on the ovi store and moreover its free.
Install the app and when you start it, you get the following screen with a Light Bulb.
Tap on the screen and light bulb glows on the screen, at the same time, the flash fires off and use it as a flash torch
Below, see the N9 Torch in action
Get it from the ovi store here
http://store.ovi.com/content/195583
