Explain that setState listener boilerplate isn’t normal.

Add section that uses AnimatedBuilder/AnimatedWidget

Refactor all DartPad samples to use non-working defaults,
& offer solution

Update vanilla bouncing ball final example to use both
upward and downward motion

“Why TickerProvider?” in animationController concepts section

How to choose between AnimatedBuilder and AnimatedWidget.

Answer question: when to use a Tween,
if you can just use upper and lower bound args for
AnimationController?
    
      when we need something other than a double?

Add margin-bottom to DartPads

Add diffs between each solution in the introduction section