Mark as prototype only after instantiating the function

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() {
@@ skipping 21 matching lines @@
 }
 
 // Test objects getting optimized as fast prototypes.
 
 function SlowPrototype() {
   this.foo = 1;
 }
 SlowPrototype.prototype.bar = 2;
 SlowPrototype.prototype.baz = 3;
 delete SlowPrototype.prototype.baz;
+new SlowPrototype;
 
 // 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__));