[ic] Use validity cells to protect keyed element stores against object's prototype chain modificati…

test/mjsunit/regress/regress-crbug-662907.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --expose-gc
+
+(function() {
+  function foo() {
+    var a = new Array();
+    a[0] = 10;
+    return a;
+  }
+
+  assertEquals(1, foo().length);
+
+  gc();
+  gc();
+  gc();
+  gc();
+
+  // Change prototype elements from fast smi to slow elements dictionary.
+  // The validity cell is invalidated by the change of Array.prototype's
+  // map.
+  Array.prototype.__defineSetter__("0", function() {});
+
+  assertEquals(0, foo().length);
+})();
+
+
+(function() {
+  function foo() {
+    var a = new Array();
+    a[0] = 10;
+    return a;
+  }
+
+  // Change prototype elements from fast smi to dictionary.
+  Array.prototype[123456789] = 42;
+  Array.prototype.length = 0;
+
+  assertEquals(1, foo().length);
+
+  gc();
+  gc();
+  gc();
+  gc();
+
+  // Change prototype elements from dictionary to slow elements dictionary.
+  // The validity cell is invalidated by making the elements dictionary slow.
+  Array.prototype.__defineSetter__("0", function() {});
+
+  assertEquals(0, foo().length);
+})();