markAsInFlight method
Mark a key as inflight. Calling this again or on a already cached entry will have no effect.
All the get function call on the same key after this will get the same result.
Implementation
Future<Null> markAsInFlight(K key) async {
if (!isKeyInFlightOrInCache(key)) {
_inflightSet[key] = InflightEntry(new Completer(), clock.now());
}
}