I'm referring specifically to the "flash" part of the flashbang (although advice on the bang part would also be appreciated).
One animal feature besides night vision I thought might be useful to splice into next generation's genetically-engineered super-soldiers is a nictitating membrane. Animals use them to keep their eyes moist and clear of dust and debris in harsh conditions without having to blink, a trait which might be useful in a desert combat environment or underwater/out at sea. Humans already have the small vestigial remains of such membrane, so I figured that with coming advances in genetic engineering, soldiers equipped with such membranes wouldn't seem too implausible a few decades from now.
But then I thought bigger. If we're already talking about giving soldiers built-in eye protection, what about protection from flashbangs and other sources of bright light, like bombs and tracer rounds? Could a nictitating membrane be further modified into a self-darkening membrane that folds over the eyes to protect them like welding goggles? Is there any natural source I could take such a self-darkening mechanism from, or would I have to augment my spliced-in membranes with some sort of artificial self-darkening filter?