Empty Array prototype elements protection needs to alert on length change.

test/cctest/test-api.cc

 // Copyright 2012 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
@@ skipping 16666 matching lines @@
   isolate1->Dispose();
 }
 
 
 TEST(VerifyArrayPrototypeGuarantees) {
   // Break fast array hole handling by element changes.
   BreakArrayGuarantees("[].__proto__[1] = 3;");
   BreakArrayGuarantees("Object.prototype[3] = 'three';");
   BreakArrayGuarantees("Array.prototype.push(1);");
   BreakArrayGuarantees("Array.prototype.unshift(1);");
+  // Break fast array hole handling by changing length.
+  BreakArrayGuarantees("Array.prototype.length = 30;");
   // Break fast array hole handling by prototype structure changes.
   BreakArrayGuarantees("[].__proto__.__proto__ = { funny: true };");
   // By sending elements to dictionary mode.
   BreakArrayGuarantees("Object.freeze(Array.prototype);");
   BreakArrayGuarantees("Object.freeze(Object.prototype);");
   BreakArrayGuarantees(
       "Object.defineProperty(Array.prototype, 0, {"
       "  get: function() { return 3; }});");
   BreakArrayGuarantees(
       "Object.defineProperty(Object.prototype, 0, {"
@@ skipping 4378 matching lines @@
 
   {
     v8::HandleScope handle_scope(isolate);
 
     // Should work
     v8::Local<v8::Object> obj = v8::Object::New(isolate);
 
     USE(obj);
   }
 }