Finds a file by class name while caching lookups to APC.
string $class A class name to resolve to file:
string|null
public function findFile($class) {
  if (false === ($file = apc_fetch($this->prefix . $class))) {
    apc_store($this->prefix . $class, $file = $this->decorated
      ->findFile($class));
  }
  return $file;
}