Change LookupForWrite to always do a full lookup and check the result.

test/mjsunit/regress/negative_lookup.js

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-Object.freeze(Object.prototype);
+function s(v) {
+  v.x = 1;
+}
+
+function c(p) {
+  return {__proto__: p};
+}
+
 var p = {};
-var o = Object.create(p);
-assertSame(p, o.__proto__);
-assertSame(p, Object.getPrototypeOf(o));
+
+// Make p the last prototype in the chain.
+p.__proto__ = null;
+
+var o1 = c(p);
+var o2 = c(p);
+var o3 = c(p);
+var o4 = c(p);
+
+// Make y go to slow mode.
+// Do this after using p as prototype, since using an object as prototype kicks
+// it back into fast mode.
+p.y = 1;
+delete p.y;
+
+// Initialize the store IC.
+s(o1);
+s(o2);
+
+// Do something with x in slow-mode p.
+Object.defineProperty(p, "x", { writable: false, value: 5 });
+
+// Verify that directly setting x fails.
+o3.x = 10;
+assertEquals(5, o3.x);
+
+// Verify that setting x through the IC fails.
+s(o4);
+assertEquals(5, o4.x);