function assertRevertLanguageCode(app) {
  assertEquals(app, lastRevertLanguageCodeCall);
  lastRevertLanguageCodeCall = null;
}
Asserts the last revertLanguageCode call was called on the specified
AuthUI instance. After assertion, the internal state counter is reset.
@param {!firebaseui.auth.AuthUI} app The AuthUI instance to check for
    revertLanguageCode calls.