Thanks for those links. I still have doubts whether tensile or flexural strength are important parameters for a small flashlight lens.
I'd say that only in relatively rare occasions a foreign object will hit the flashlight lens directly. The majority of cases will be that the flashlight "just" falls onto the hard ground and the resulting G-forces are causing the shattering (-> brittleness ("a material is brittle if, when subjected to stress, it breaks without significant deformation (strain)")).
Or, by deformation of the flashlights bezel, the metal will "poke" into the lens and cause it to fracture. Don't know what kind of physical force measures the latter example but I would say usually there are no forces flexing or pulling your (~1") flashlight lens so these might not be the correct forces to compare the different materials by.
Dropping the light would probably be an example of compressive force, from the above links sapphire is still stronger. There would also be a flexural component depending on how it hits the ground.
The forces from poking the lens will definitely exert a flexural force on the lens. To make a example, get a sheet of paper, hold it up by two sides and poke it somewhere. The paper will bend (read: flex) around the point that you're poking it at. This is the same deal for any object being poked regardless of material or size, it's just that it's a lot more obvious with the sheet of paper. You can also test the same if the flashlight is dropped by holding the paper at whatever angle you choose and lowering it onto the ground. You'll see the paper buckle and/or bend, indicating that there is almost always a flexural component, and a compressive component as well for that matter.
Even when it starts to break, in crack propagation, as per fyrstormer's first link sapphire also has a higher fracture toughness than glass.
Note that while the definition of brittleness in your post is correct, it indicates nothing about the overall strength of the material. Silly putty is much more flexible than glass or sapphire, but I'd bet on the glass or sapphire withstanding more than the putty would, even taking the putty's deformation into account. Practically, I don't care about whether the material flexes or not, all I care about is that it can withstand the forces without breaking or going into plastic deformation.