code: function getItemTextContents(element) {
    if (!element[itemTextContentsSymbol]) {
      const items = element.items;
      element[itemTextContentsSymbol] = items.map(child => {
        const text = child.textContent || child.alt;
        return text.toLowerCase();
      });
    }
    return element[itemTextContentsSymbol];
  }
docs: Return an array of the text content (in lowercase) of all items. Cache these results.