Pour petrol onto it. Take a match and light it on fine. You have a burning flashlight.
Seriously, flashlight's primary purpose is illuminating stuff. So, visible light. (Exceptions are specialty flashlights, e.g. UV light)
To burn stuff, you don't need visible light. So infrared is fine.
But wait, what is a source of a lot of infrared radiation? Exactly, incandescent lights (halogen and HID).
High powered LEDs should be fine too, but they need good cooling.
Fun stuff: the heat you feel from a campfire isn't hot air, it's infrared radiation. You can check that by holding something in front of your hand (for example, your other hand) to absorb the radiation. In that case, you won't feel the heat anymore.