Creates a flipped TweenSequence.
items parameter must be a list of one or more TweenSequenceItems.
There's a small cost associated with building a
TweenSequence so it's
best to reuse one, rather than rebuilding it on every frame, when that's
FlippedTweenSequence(List<TweenSequenceItem<double>> items) : assert(items != null), super(items);