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_(unwrap_jso(this), unwrap_jso((currentUsageInBytes, currentQuotaInBytes) => usageCallback(currentUsageInBytes, currentQuotaInBytes)), unwrap_jso((error) => errorCallback(wrap_jso(error))));
    return;
  }
  _blink.BlinkDeprecatedStorageQuota.instance.queryUsageAndQuota_Callback_1_(unwrap_jso(this), unwrap_jso((currentUsageInBytes, currentQuotaInBytes) => usageCallback(currentUsageInBytes, currentQuotaInBytes)));
  return;
}