function isElementUpgraded_(element, jsClass) {
    var upgradedList = getUpgradedListOfElement_(element);
    return upgradedList.indexOf(jsClass) !== -1;
  }
Returns true if the given element has already been upgraded for the given
class.

@param {!Element} element The element we want to check.
@param {string} jsClass The class to check for.
@returns {boolean}
@private