Frodo's dream (and the dozens of other similar examples) are different, Gorwingel. That's an example of foreshadowing, a device used frequently by JRRT inside the story -
see this thread for more.
Merry's scar and the other examples here are really spoilers that are outside the story. This didn't come out as clear as I thought it would, but hopefully you can see what I'm getting at. I guess the main difference IMO is that references such as Merry's scar were not meant as foreshadowing later events as such. They merely explain a fact to us, rather than describe a later episode. They seem almost unintentional.