Avoid inheritance/overriding in EntityStore#getRootIdSet.
Avoid inheritance/overriding in EntityStore#getRootIdSet.

Explicit instanceof checks are so much more readable and less surprising
than multiple method implementations spread across different classes.

This implementation also avoids creating multiple Set objects when there
are any active optimistic layers.

apollostack/apollo-client,apollographql/apollo-client,apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client