void queryUsageAndQuota(StorageUsageCallback usageCallback(int currentUsageInBytes, int currentQuotaInBytes), [StorageErrorCallback errorCallback(DomError error)])

Source 

void queryUsageAndQuota(StorageUsageCallback usageCallback, [StorageErrorCallback errorCallback]) {
  if (errorCallback != null) {
    _blink.BlinkDeprecatedStorageQuota.instance.queryUsageAndQuota_Callback_2_(this, usageCallback, errorCallback);
    return;
  }
  _blink.BlinkDeprecatedStorageQuota.instance.queryUsageAndQuota_Callback_1_(this, usageCallback);
  return;
}