js-on-sx: delete obj.key actually removes the key

js-delete-prop was setting value to js-undefined instead of
removing the key, so 'key' in obj remained true and proto-chain
lookup didn't fall through. Switched to dict-delete!.
Now delete Boolean.prototype.toString; Boolean.prototype.toString()
walks up to Object.prototype.toString and returns "[object Boolean]".
built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,45 +1,45 @@
 {
   "totals": {
     "pass": 44,
-    "fail": 3,
+    "fail": 6,
     "skip": 0,
-    "timeout": 3,
+    "timeout": 0,
     "total": 50,
     "runnable": 50,
     "pass_rate": 88.0
   },
   "categories": [
     {
-      "category": "built-ins/Number",
+      "category": "built-ins/Object",
       "total": 50,
       "pass": 44,
-      "fail": 3,
+      "fail": 6,
       "skip": 0,
-      "timeout": 3,
+      "timeout": 0,
       "pass_rate": 88.0,
       "top_failures": [
         [
-          "Timeout",
-          3
+          "ReferenceError (undefined symbol)",
+          4
         ],
         [
-          "Test262Error (assertion failed)",
-          3
+          "SyntaxError (parse/unsupported syntax)",
+          2
         ]
       ]
     }
   ],
   "top_failure_modes": [
     [
-      "Timeout",
-      3
+      "ReferenceError (undefined symbol)",
+      4
     ],
     [
-      "Test262Error (assertion failed)",
-      3
+      "SyntaxError (parse/unsupported syntax)",
+      2
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 110.0,
+  "elapsed_seconds": 32.3,
   "workers": 1
 }