http://docs.mongodb.org/manual/release-notes/2.6/#aggregation-operation-may-return-a-cursor mentions "the temporary mongo shell helper db.collection.aggregateCursor() provides access to this capability"

That temporary helper has been removed as of https://github.com/mongodb/mongo/commit/061a086641a2638d73884ad573eaea2a3ab8459f (2.5.3)

The old .aggregate() method now has split personality:

1. Old varargs of pipeline ops
2. An array of pipeline ops as first argument, second argument "options" (such as {allowDiskUsage: true}

   )