CSS class plus Flexi attributes are broke

[gtb104]

With the 3.0.0-alpha.0 branch, the following code:

function updateClassNode(...) {
  ...
  else {
    // classNode.value.chars = `${classNode.value.chars} ${classNames.join(' ')}`;
  }
  ...

Breaks this code <box xs="3" class="something"></box>

This else block is commented out, so it never appends the generated class names to an existing class string.