Using a microcontroller to make a series of LEDs blink one after the other
With 5 LEDs, 1 button, and 2 pins for power this is a perfect match for an 8-pin micro-controller. The hobby-friendly DIP-8 package would easily fit in your bike light, but the manufacturer probably used a cheaper un-packaged chip directly bonded to the PCB and put a blob of black goo on it.  This manufacturing method is sometimes called "chip on board".