Implement FlippedTween

We should provide a `FlippedTween`, like `ReverseTween`. For example, something like:

```
class FlipTween<T> extends Tween<T> {
  FlipTween(this.parent)
    : assert(parent != null),
      super(begin: parent.end, end: parent.begin);

  final Tween<T> parent;

  @override
  T lerp(double t) => 1.0 - parent.lerp(1.0 - t);
}
```