function getUpgradedListOfElement_(element) {
    var dataUpgraded = element.getAttribute('data-upgraded');
    // Use `['']` as default value to conform the `,name,name...` style.
    return dataUpgraded === null ? [''] : dataUpgraded.split(',');
  }
Returns an array of the classNames of the upgraded classes on the element.

@param {!Element} element The element to fetch data from.
@return {!Array<string>}
@private