[es6] Partially implement Reflect.preventExtensions.

test/mjsunit/object-prevent-extensions.js

 // Copyright 2010 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 70 matching lines @@
 assertEquals(undefined, obj2[1]);
 
 var arr = new Array();
 arr[1] = 10;
 
 Object.preventExtensions(arr);
 
 arr[2] = 42;
 assertEquals(10, arr[1]);
 
-// We should still be able to change exiting elements.
+// We should still be able to change existing elements.
 arr[1]= 42;
 assertEquals(42, arr[1]);
 
 
 // Test the the extensible flag is not inherited.
 var parent = {};
 parent.x = 42;
 Object.preventExtensions(parent);
 
 var child = Object.create(parent);
@@ skipping 51 matching lines @@
 obj = { prop1: 1, prop2: 2, 75: 'foo' };
 obj2 = { prop1: 3, prop2: 4, 150: 'bar' };
 assertTrue(%HaveSameMap(obj, obj2));
 Object.preventExtensions(obj);
 Object.preventExtensions(obj2);
 assertFalse(Object.isExtensible(obj));
 assertFalse(Object.isExtensible(obj2));
 assertFalse(Object.isSealed(obj));
 assertFalse(Object.isSealed(obj2));
 assertTrue(%HaveSameMap(obj, obj2));