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