CSS pruning regression: class: directives not recognized by unused selector detection

[Gitkubikon]

Describe the bug

After upgrading to Svelte 5.x (specifically after PR #16179) the CSS pruning algorithm incorrectly flags CSS selectors as unused when they target classes applied via class: directives.

Reproduction

Component code:

<script>
  let collapsed = false;
</script>

<div class="sidebar" class:collapsed={collapsed}>
  <div class="content">Content</div>
</div>

<style>
  .sidebar.collapsed {
    width: 60px; /* This selector is flagged as unused */
  }
  
  .collapsed .content {
    opacity: 0; /* This selector is also flagged as unused */
  }
</style>

### Logs

```shell
Unused CSS selector ".sidebar.collapsed"
https://svelte.dev/e/css_unused_selector

System Info

System:
    OS: Linux 6.15 Archcraft
    CPU: (12) x64 AMD Ryzen 5 7600X 6-Core Processor
    Memory: 42.22 GB / 62.02 GB
    Container: Yes
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.19.2 - /usr/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 11.4.2 - /usr/bin/npm
    pnpm: 10.12.1 - /usr/bin/pnpm
    bun: 1.2.16 - ~/Projects/********/********/node_modules/.bin/bun
  Browsers:
    Chromium: 137.0.7151.103
  npmPackages:
    svelte: 5.34.6 => 5.34.6

Severity

blocking an upgrade

[laszlokorte]

I just stumbled upon the same issue and noticed that it works when writing class={"sidebar"} instead of class="sidebar"

[Gitkubikon]

I just stumbled upon the same issue and noticed that it works when writing class={"sidebar"} instead of class="sidebar"

Your workaround of using class={"sidebar"} essentially forces the algorithm into the "can't be sure, better keep it" mode, I already proposed a PR #16202 wich fixes the issue, so don't bother rewriting your whole code base :)

[mattpilott]

I added this comment to the pr, adding here also for visibility.
5.34.6 is also breaking sites where class props are defined as undefined:
let { mypropclass = undefined } = $props()
<div class:mypropclass></div>