In the forward to the third edition of his Undergraduate Algebra, Lang mentions:
A new section in Chapter IV gives a complete account of the Mason-Stothers theorem about polynomials, with Noah Snyder's beautifully simple proof. It is worth emphasizing that the derivative for polynomials is a purely algebraic operation, for which limits are not required. A Springer pamphlet has been published to present a self-contained treatment of polynomials (from scratch) culminating with this topic.
What publication is he referring to?
Perhaps it is the following article (at least, Serge Lang is talking about it):
Noah Snyder An alternate proof of Mason’s theorem, Elem. Math. 55 (2000), 93–94; Springer Verlag.
This gives a ‘proof from the Book’ for Mason’s ABC theorem.