private bool IsStable(string packageId, Version packageVersion)
        {
            bool isStable;

            if (_stablePackageVersions != null)
            {
                Version stableVersion;
                isStable = _stablePackageVersions.TryGetValue(packageId, out stableVersion) && stableVersion >= packageVersion;
            }
            else
            {
                isStable = _index.IsStable(packageId, packageVersion);
            }

            return(isStable);
        }