Here's the quote with difficulties marked in bold

(The priest) "but I saved the cross, as the cross will always be saved. It is at Westminster by now. I rather wonder you didn't stop it with the Donkey's Whistle."

"With the what?" asked Flambeau. (the criminal)
"I'm glad you've never heard of it," said the priest, making a face. "It's a foul thing. I'm sure you're too good a man for a Whistler. I couldn't have countered it even with the Spots myself; I'm not strong enough in the legs."
"What on earth are you talking about?" asked the other. "Well, I did think you'd know the Spots," said Father Brown, agreeably surprised.

My question is what could be Donkey's Whistle, and Whistler and Spots.