How do I re-establish the crop dial in photos?

The dial is no longer there in Photos 8, but you can use the straighten slider or can turn the photo with two fingers on the trackpad. Apple has changed the controls on the Mac to match the sliders (straighten, horizontal, vertical)on the iPhone and iPad.

Some external editors are still offering the nice, old straighten dial, for example Luminar Neo.

Honestly, the straighten tool isn't as awful as I first thought.

It seems shorter as though it offers less control, but it seems to have about the same resolution as the wheel. I still like the wheel better, but I remember only moving my cursor up and down, anyway. It's not so bad...

In Luminar Neo the Straighten Tool is in the Crop tool.

Click to the right of the cropping rectangle, hold down on the Trackpad, and a curved arrow will appear. Then you can drag up or down to straighten the image, while the angle is shown.

Apple changed the controls around. See this: Crop and straighten photos in Photos on Mac - Apple Support

Well, you'd really like to have the machines adapt to us rather than vice-versa!