[cross-context] create new function prototypes in the context of the function

test/mjsunit/cross-realm-filtering.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.
 
 var realms = [Realm.current(), Realm.create()];
 
 // Check stack trace filtering across security contexts.
 var thrower_script =
     "(function () { Realm.eval(Realm.current(), 'throw Error()') })";
 Realm.shared = {
@@ skipping 52 matching lines @@
   Realm.shared.caller_1(f_1);                                                  \
 ";
 
 Realm.eval(realms[1], script);
 assertSame(null, Realm.shared.result_0);
 assertSame(Realm.shared.caller_1, Realm.shared.result_1);
 
 Realm.eval(realms[0], script);
 assertSame(Realm.shared.caller_0, Realm.shared.result_0);
 assertSame(null, Realm.shared.result_1);
+
+
+// test that do not pollute / leak a function prototype v8/4217
+var realmIndex = Realm.create();
+var otherObject = Realm.eval(realmIndex, "Object");
+
+var f = Realm.eval(realmIndex, "function f(){}; f");
+f.prototype = null;
+
+var o = new f();
+var proto = Object.getPrototypeOf(o);
+assertFalse(proto === Object.prototype);
+assertTrue(proto === otherObject.prototype);
+
+o = Realm.eval(realmIndex, "new f()");
+proto = Object.getPrototypeOf(o);
+assertFalse(proto === Object.prototype);
+assertTrue(proto === otherObject.prototype);