IOFileSystem - problem using the Customize example in the docs for 2.0.0

I'm upgrading from the previous version to 2.0.0 and trying to follow the new Customize instructions to use CacheManager instead of BaseCacheManager. The reference to IOFileSystem doesn't resolve. What am I doing wrong?

```dart
CacheManager(
Config(
key,
stalePeriod: const Duration(days: 7),
maxNrOfCacheObjects: 60,
repo: JsonCacheInfoRepository(databaseName: key),
fileSystem: IOFileSystem(key),
fileService: HttpFileService(),
),
);
```