Was always emitting comma-joined via js-list-join, so user
mutations of Array.prototype.toString had no effect on String(arr)
/ "" + arr. Now look up the override via js-dict-get-walk and call
it on the list as this; fall back to (js-list-join v ",") when the
override doesn't return a string.
String fail count: 11 → 9. conformance.sh: 148/148.