store(accounts, opt_config) {
    if (typeof this.config_['callbacks']['store'] === 'function') {
      this.config_['callbacks']['store'](
          undefined, firebaseui.auth.acClient.DummyApi.UNAVAILABLE_ERROR_);
    }
  }
Stores the accounts. The callback is always invoked with a service
unavailable error.

@param {Array<Object>} accounts The accounts to store.
@param {Object=} opt_config The optional client configuration.