Here’s a very simple Flash flashlight effect that uses almost no ActionScript. The only AS used is for rotation and key detection.
The effect is archived with a mask and a filter. The filter is a simple gradient fill, same color as the “dark”. One side the alpha is 100, the other size alpha 0. Both the mask and the filter are the same shape and size (I’m using the same movieclip actually). To make everything work, you have to rotate and move the filter and mask at the same time (along with the player).
Download Fla Sample
Also note the center registration point for the filter, mask and player. Make sure everything is aligned or the effect will break.