datastore: Add DecodeCursor to [Raw]Interface.

impl/prod/raw_datastore.go

Index: impl/prod/raw_datastore.go
diff --git a/impl/prod/raw_datastore.go b/impl/prod/raw_datastore.go
index 8988f792a6cf176007f2fc1e7d4c30dd98944609..bb8a37779577bd5edb7186e494818e171734a87b 100644
--- a/impl/prod/raw_datastore.go
+++ b/impl/prod/raw_datastore.go
@@ -131,6 +131,10 @@ func (d rdsImpl) NewQuery(kind string) ds.Query {
 	return queryImpl{datastore.NewQuery(kind)}
 }
 
+func (d rdsImpl) DecodeCursor(s string) (ds.Cursor, error) {
+	return datastore.DecodeCursor(s)
+}
+
 func (d rdsImpl) Run(q ds.Query, cb ds.RawRunCB) error {
 	tf := typeFilter{}
 	t := q.(queryImpl).Query.Run(d)