What I've done to fix this:

- add a sclang-class-list variable in sclang-language.el, which is a
  list of all the classes known to sclang. this is populated when
  sclang starts.
- update sclang-font-lock-class-keyword-matcher in
  sclang-mode.el. since the sclang-class-name-regexp now will match
  all words that start with a capital letter (see next bullet point
  for that change), this function had to be updated to check to make
  sure that the word starting with a capital letter is in the list of
  classes. if it is, then we know it's a class and it gets
  highlighted. if it's not, then it's just something the user typed
  with a capital letter, so we don't highlight it.
- update sclang-update-font-lock in sclang-mode.el. instead of
  generating a huge regexp from a list of all the classes in
  SuperCollider, just run the normal fontification from that
  function. this avoids making the regexp too big and thus prevents
  fontification from failing.