function assertResendCountdownIsHidden(component, isHidden) {
  var el = component.getResendCountdown();
  assertEquals(isHidden, goog.dom.classlist.contains(el, 'firebaseui-hidden'));
}
Asserts the resend countdown is hidden.
@param {!goog.ui.Component} component The PhoneSignInFinish component.
@param {boolean} isHidden True to assert hidden; false to assert visible.