[builtins] DeleteProperty: Handle last-added fast properties

test/mjsunit/dictionary-properties.js

 // Copyright 2014 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
 
 // Test loading existent and nonexistent properties from dictionary
 // mode objects.
 
 function SlowObject() {
   this.foo = 1;
   this.bar = 2;
   this.qux = 3;
+  this.z = 4;
   delete this.qux;
   assertFalse(%HasFastProperties(this));
 }
 function SlowObjectWithBaz() {
   var o = new SlowObject();
   o.baz = 4;
   return o;
 }
 
 function Load(o) {
   return o.baz;
 }
 
 for (var i = 0; i < 10; i++) {
   var o1 = new SlowObject();
   var o2 = SlowObjectWithBaz();
   assertEquals(undefined, Load(o1));
   assertEquals(4, Load(o2));
 }
 
 // Test objects getting optimized as fast prototypes.
 
 function SlowPrototype() {
   this.foo = 1;
 }
 SlowPrototype.prototype.bar = 2;
 SlowPrototype.prototype.baz = 3;
+SlowPrototype.prototype.z = 4;
 delete SlowPrototype.prototype.baz;
 assertFalse(%HasFastProperties(SlowPrototype.prototype));
 var slow_proto = new SlowPrototype;
 // ICs make prototypes fast.
 function ic() { return slow_proto.bar; }
 ic();
 ic();
 assertTrue(%HasFastProperties(slow_proto.__proto__));
 
 // Prototypes stay fast even after deleting properties.
 assertTrue(%HasFastProperties(SlowPrototype.prototype));
 var fast_proto = new SlowPrototype();
 assertTrue(%HasFastProperties(SlowPrototype.prototype));
 assertTrue(%HasFastProperties(fast_proto.__proto__));