function Ball(center) {
   this.center = center;
   this.growPerSec = Ball.GROW_PER_SEC;
   this.radius = this.growPerSec * 2;
   this.color = 0;
   return this;
}
@fileoverview Minimal is the smalles GameJs app I could think of, which still shows off
most of the concepts GameJs introduces.

It's a pulsating, colored circle. You can make the circle change color
by clicking.