Smarter CSS encapsulation with concatenated classes

[Rich-Harris]

In a case like this...

<div class="thing {active ? 'active' : ''}">
  some stuff
</div>

<style>
  .thing {...}
  .active {...}
  .thing.active {...}

  .unused {...}
</style>

...Svelte bails out of trying to figure out which classes could apply to the div, and ends up including all the classes in the <style> block, including the unused one. It should definitely be able to figure out that only the first three could exist in the markup.