cancelSingletonSubscription method
Cancels StreamSubscription associated with singletonIdentifier.
Implementation
bool cancelSingletonSubscription(Object singletonIdentifier,
    [bool singletonIdentifyByInstance = true]) {
  var signature =
      _getListenSignature(singletonIdentifier, singletonIdentifyByInstance);
  if (signature != null) {
    _listenSignatures.remove(signature);
    return signature.cancel();
  }
  return false;
}