ThemeRegistry::has

public ThemeRegistry::has($key)

public function has($key) {
  // Since the theme registry allows for theme hooks to be requested that
  // are not registered, just check the existence of the key in the registry.
  // Use array_key_exists() here since a NULL value indicates that the theme
  // hook exists but has not yet been requested.
  return array_key_exists($key, $this->storage);
}