Analyse ternary expressions when detecting unused CSS

[Rich-Harris]

In 1.24, Svelte will warn on unused selectors, but cases like this result in a false negative:

<button class='{{active ? "active" : "inactive"}}'>click me</button>

<style>
  .active {...}
  .inactive {...}
  .irrelevant {...}
</style>

It's actually not very hard to determine that the button has two possible classes — active and inactive — and optimise accordingly.

[PaulBGD]

What if the class is computed by a compute function?

[Rich-Harris]

In some cases we could do it:

export default {
  computed: {
    buttonClass(active) {
      return active ? 'active' : 'inactive';
    }
  }
};

It would require a bit more machinery, but we can determine that the return value of buttonClass must be either active or inactive.

In some cases it would be much harder:

import buttonClass from './buttonClass.js';

export default {
  computed: {
    buttonClass(active) {
      return buttonClass(active);
    }
  }
};

The same goes for helper functions. Inline expressions are definitely easier though.

[Rich-Harris]

Inline expressions are now handled. Have left computed/helper equivalents at least for the time being