As far as I understand it, Unity have, since 4 tried to begin to use, "proper" clean, legal code, where as before some of their stuff was a bit lazily coded, because "well if Unity Engine works, why worry"... now they are tightening up the syntax, so now, slightly dodgy code will throw a warning, encouraging you to tidy up, but soon, it may throw a warning.
Would be nice if they tidied up their OWN code though... but as this file was part of the old Character Controller stuff... I guess this is another example of them trying to push Mechanim on you... Im sure I go to it someday, but I really wished they had overhauled the OLD animation system, cos I liked it, it just had some annoyances (with cutting and pasting keyframes, or dragging.. etc)