What makes code tree-shakeable, how do bundlers use ESM static structure and the sideEffects flag, and why do barrel files and class decorators defeat dead-code elimination?

Imagine you have a big box of LEGO bricks, but you only need a few to build a small car. Tree shaking is like a robot that picks out only the bricks you need and throws away the rest, so your toy is lighter and faster. But if some bricks are glued together or have special magic, the robot can't separate them, so you have to keep them all.