js-on-sx: js-dict-get-walk falls back to Object.prototype

Object literals didn't carry a __proto__ link, so ({}).toString()
couldn't reach Object.prototype.toString. Added a cond clause: if
the object has no __proto__ AND is not Object.prototype itself,
walk into Object.prototype. Now ({}).toString() works, override
of Object.prototype.toString propagates, and ({a:1}).hasOwnProperty
('a') returns true. built-ins/String: 69/99 → 71/99 (canonical),
71/99 → 74/99 (isolated). conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 69,
-    "fail": 24,
+    "pass": 71,
+    "fail": 22,
     "skip": 1,
     "timeout": 6,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 69.7
+    "pass_rate": 71.7
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 69,
-      "fail": 24,
+      "pass": 71,
+      "fail": 22,
       "skip": 1,
       "timeout": 6,
-      "pass_rate": 69.7,
+      "pass_rate": 71.7,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          12
+          10
         ],
         [
           "TypeError: not a function",
@@ -44,7 +44,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      12
+      10
     ],
     [
       "TypeError: not a function",
@@ -68,6 +68,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 344.4,
+  "elapsed_seconds": 375.3,
   "workers": 1
 }