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

test/mjsunit/regress/readonly4.js

Index: test/mjsunit/regress/readonly4.js
diff --git a/test/mjsunit/regress/regress-2565.js b/test/mjsunit/regress/readonly4.js
similarity index 71%
copy from test/mjsunit/regress/regress-2565.js
copy to test/mjsunit/regress/readonly4.js
index a77806a62e2209d355fce393b98797f1d20f40f8..7e0c141cf5e4582a6eee5fea60141f7b6d7589bb 100644
--- a/test/mjsunit/regress/regress-2565.js
+++ b/test/mjsunit/regress/readonly4.js
@@ -25,8 +25,48 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-Object.freeze(Object.prototype);
+var slow = {};
 var p = {};
-var o = Object.create(p);
-assertSame(p, o.__proto__);
-assertSame(p, Object.getPrototypeOf(o));
+
+slow.__proto__ = p;
+slow.x = 10;
+slow.y = 10;
+Object.defineProperty(p, "x", {writable:false, value:5});
+
+function c(p) {
+  return {__proto__: p};
+}
+
+function s(v) {
+  return v.x = 1;
+}
+
+function s_strict(v) {
+  "use strict";
+  v.x = 1;
+}
+
+var o1 = c(slow);
+var o2 = c(slow);
+var o1_strict = c(slow);
+var o2_strict = c(slow);
+var o3 = c(slow);
+var o4 = c(slow);
+
+// Make s slow.
+delete slow.y;
+
+s(o1);
+s(o2);
+s_strict(o1_strict);
+s_strict(o2_strict);
+
+delete slow.x;
+// Directly setting x should fail.
+o3.x = 20
+assertEquals(5, o3.x);
+
+// Setting x through IC should fail.
+s(o4);
+assertEquals(5, o4.x);
+assertThrows("s_strict(o4);");