A few improvements

[jimmyff]

This solves a few issues:

1. SizedBox with height:30 made this unusable for small font sizes
2. Changing the container height seemed like a strange way of doing an animation and wasn't working for me, maybe as I was using a really small font. Anyway -replacing this with Transform.translate is a more standard way of achieving this
3. endTweenValue being fixed as 8 means that really small fonts had a huge animation and really big fonts had a small animation, now it's calculated based on fontSize
4. Solved all of package:pedantic warnings & notices for this single file (I didn't do the whole project).