The jstests/core/index_key_expression.js failed in some of the sharding passthrough with the error - "The field '$const' is not valid for an index specification.", where the extra $const specifier gets prepended to the index spec in some cases, as in: 

{ $const: { key: { b: 1.0, a: -1.0 }, name: \"btreeIndex\" } }

The  $const causes the index-specification validation code to flag this spec as invalid; hence, the JS test fails with an unexpected error. This could be a legit issue and this ticket is about investigating the cause.