js-get-prop for SX lists fell through to js-undefined for any key
not in its hardcoded method list, so Array.prototype.myprop and
Object.prototype.hasOwnProperty were invisible to arrays.
Switched the fallback to walk Array.prototype via js-dict-get-walk,
which already chains to Object.prototype.
built-ins/Array: 14/45 → 16/45. conformance.sh: 148/148.