The $facet stage needs to override DocumentSource::needsPrimaryShard()|https://github.com/mongodb/mongo/blob/r3.3.11/src/mongo/db/pipeline/document_source.h#L221 to forward calls and see if any sub-stage needs the primary shard.

This can cause $facet stages containing a $lookup to run on a shard that does not have the collection being looked up from (as discovered when running jstests/aggregation/bugs/server19095.js through the passthrough suite from SERVER-25757)