Changes to /tools/dart-format for new formatter

[munificent]

Describe the problem

There are a few changes to /tools/dart-format that we'll probably want to make because of the new formatting style that's coming out in 3.7. They are:

Expected fix

If you specify a directory, dart format affects only the files in the immediate directory; it doesn't recurse through subdirectories.

Uh, that's actually never been true. 🙃 If you pass a directory path, it recurses into its subdirectories as well. (Otherwise dart format . wouldn't do anything very useful since almost all of your Dart files are in lib/, bin/, etc.)

Use trailing commas

We should delete this entire section. With the new style, it adds and removes trailing commas on your behalf, and you basically always get the style that explicit trailing commas would give you before.

Affects whitespace only

Technically, it will now also make some non-whitespace changes because it will add and remove trailing commas, and may move a comment before or after a comma.

I don't know if that's splitting hairs or not, but if we want to be completely accurate, it would be good to re-word this or maybe just remove this section.