This kata is a bit problematic to understand since the examples and question raise the obvious situation in which all the keys are 'a' and the key that is passed is 'b'... what happens then?
What is the use in the 'key' argument passed to this method?

I fixed the test cases and added a 2nd example to try to drive the point home.

Crap. I created this kata before the "show test cases" feature existed. Thanks for pointing that out.

Good point. I was originally thinking that anyone could technically look at the test cases and then just reproduce the expected result, but I guess if there was more than one example then it would less trivial to cheat.

Cheat how? By looking at the test cases on the discuss page before solving it?