Add -w shorthand for --watch CLI flag

[henrycatalinismith]

Hey there! I noticed the help-wanted label on #1145 and thought it might be a nice way to learn how to contribute to one of my fav open source projects, so I've had a try at it. Hopefully I've done a decent job here but no stress if not and you need to reject or point me at some extra steps, or if you're just a little too busy for this kind of thing right now! 😇

The code change here was really tiny. I looked into adding some tests in test/cli/shared/watch.dart but it looks like the shorthand functionality is coming from ArgParser so I wasn't sure how much of the existing --watch tests you'd wanna duplicate for verifying that. I've signed the CLA and checked the branch with pub run grinder, dartanalyzer lib test and pub run test -x node so I think I covered those bases anyway. Here's how it looks in the example output when I do dart2native -o bin/sass bin/sass.dart && ./bin/sass. Seems to come through automatically, which is nice!

━━━ Other ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
-w, --watch                    Watch stylesheets and recompile when they change.
    --[no-]poll                Manually check for changes rather than using a native watcher.
                               Only valid with --watch.

I've done some manual testing locally too and it's working great. Oh and I have a sass-site commit ready documenting the new shorthand in case you're keen to have that right away. It occurred to me though that you might also wanna delay shipping that for a moment or two to give the new flag a chance to propagate among the install base first.

Let me know if there's a specific bit of automated testing you'd like me to add, or if I've just completely missed the mark here! It's been fun getting this far and I'm happy to do a bit of extra work on this PR or close it unmerged if necessary!

[henrycatalinismith]

Oh interesting, one of the test failures looked like it might have been caused by my change here when I spotted the filename, so I was worried for a second.

00:31 +0 -6: [VM] test\cli\node\watch_test.dart: (setUpAll) [E]
182
  IsolateSpawnException: Unable to spawn isolate: Error: Cannot run with sound null safety, because the following dependencies
183
  don't support null safety:

I looked into some of the other PRs in the queue though and found the same error on the "Fix the name of the blackness argument" PR so I don't think I've broken that. I've had a try at fixing some of the dartanalyzer errors breaking all the builds BTW and most of them seem to be within my abilities except for a few tricky ones I'm not enough of a Dart pro for yet. I'd be happy to throw up a PR with the easy wins if you want though!

[nex3]

Thanks for the contribution! You're right that the test failures are unrelated; we should be resolving those soon. In the meantime, can you add an entry describing this change to the changelog?

[henrycatalinismith]

Done! I've added it to the 1.32.9 section after having a look through the repo history and seeing that that seems to be A Thing here for teeny tiny changes like these when there are already new minor versions awaiting publishing. No trouble at all to move this into a new version of its own though if you'd prefer that instead! Can see there's a merge conflict incoming there with the minimum Chokidar version change so there's a good chance I'll be updating this branch again either way.