We would like to move away from system.js for new Javascript operators such as $function and $accumulator. But for backwards compatibility we need to continue to support it for $where and MapReduce, even though these will desugar to $function and $accumulator.

The switch should be per-expression, not per query: in a query containing both $where and $function, the $where expression should see system.js functions but the $function expression should not.