Add Kind/StringID/IntID back to Key.

impl/memory/datastore_data.go

Index: impl/memory/datastore_data.go
diff --git a/impl/memory/datastore_data.go b/impl/memory/datastore_data.go
index 96c85f8af98a3ca3eefc7ea20beecf485beb5c03..f563b4ba1e230c5b789facb5c1f2b45ea1485470 100644
--- a/impl/memory/datastore_data.go
+++ b/impl/memory/datastore_data.go
@@ -143,7 +143,7 @@ func (d *dataStoreData) allocateIDs(incomplete *ds.Key, n int) int64 {
 func (d *dataStoreData) allocateIDsLocked(ents *memCollection, incomplete *ds.Key, n int) int64 {
 	idKey := []byte(nil)
 	if incomplete.Parent() == nil {
-		idKey = rootIDsKey(incomplete.Last().Kind)
+		idKey = rootIDsKey(incomplete.Kind())
 	} else {
 		idKey = groupIDsKey(incomplete)
 	}
@@ -153,7 +153,7 @@ func (d *dataStoreData) allocateIDsLocked(ents *memCollection, incomplete *ds.Ke
 func (d *dataStoreData) fixKeyLocked(ents *memCollection, key *ds.Key) *ds.Key {
 	if key.Incomplete() {
 		id := d.allocateIDsLocked(ents, key, 1)
-		key = ds.NewKey(key.AppID(), key.Namespace(), key.Last().Kind, "", id, key.Parent())
+		key = ds.NewKey(key.AppID(), key.Namespace(), key.Kind(), "", id, key.Parent())
 	}
 	return key
 }