revalidatePurgedEntries method
Restore purged entries that are currently valid. Only relevant if keepPurgedEntries is true.
Implementation
int revalidatePurgedEntries() {
  var purged = _purged;
  if (purged == null) return 0;
  var revalidateCount = 0;
  purged.removeWhere((k, v) {
    var value = v.value;
    if (isValidEntry(k, value)) {
      ++revalidateCount;
      _map[k] = value;
      return true;
    } else {
      return false;
    }
  });
  if (revalidateCount > 0) {
    _expireCache();
  }
  _revalidatedPurgedEntriesCount += revalidateCount;
  return revalidateCount;
}