Add TODOs.
Rename @method_calls to @signatures
Rename _check_for_duplicate_signature to _check_if_duplicate_signature
Rename _state_in to _is_state_in
Replace @current_signature with _current_signature()
Extract method _current_method_name()
Only need to create a given mock method once, not once per .expects
mock() now reports all mock errors, not just the first one with errors.
Replace 'throw new Error(msg)' with 'throw msg'
Add class, method, and function comments.