Because of this new package I needed, I had to upgrade my Flutter. So I ran flutter upgrade as usual. This time instead of upgrading without a hitch, it told me:

% flutter upgrade
Upgrading Flutter from ~/flutter...
From https://github.com/flutter/flutter
...
fatal: Not possible to fast-forward, aborting.

So I went to the folder and ran git status and sure enough something had happened:

flutter % git status
On branch beta
Your branch and 'origin/beta' have diverged,
and have 2 and 467 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)

nothing to commit, working tree clean

Here's the view in SourceTree:

I am not sure why it diverged though. I ran flutter channel beta according to the docs, but that failed too:

% flutter channel beta
Switching to flutter channel 'beta'...
git: Already on 'beta'
git: Your branch and 'origin/beta' have diverged,
git: and have 2 and 467 different commits each, respectively.

To manually reset the repo back to origin/beta I ran:

% git reset --hard origin/beta 
HEAD is now at 2738a1148b 1.18.0-11.1.pre beta cherrypicks (#57058)

This time, flutter upgrade worked. Still not sure how it ever diverged in the first place though.