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

test/mjsunit/regress/readonly1.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
 //       with the distribution.
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 //       from this software without specific prior written permission.
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 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (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);
+function s(v) {
+  v.x = 1;
+}
+
+function s_strict(v) {
+  "use strict";
+  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));
+
+var o1 = c(p);
+var o2 = c(p);
+var o3 = c(p);
+var o4 = c(p);
+
+// Make p go slow.
+p.y = 1;
+delete p.y;
+p.x = 5;
+
+// Initialize the store IC.
+s(o1);
+s(o2);
+s_strict(o1);
+s_strict(o2);
+
+// Make x non-writable.
+Object.defineProperty(p, "x", { writable: false });
+
+// Verify that direct setting fails.
+o3.x = 20;
+assertEquals(5, o3.x);
+
+// Verify that setting through the IC fails.
+s(o4);
+assertEquals(5, o4.x);
+assertThrows("s_strict(o4);");