Fix initial prototype of WeakMap function.

test/mjsunit/harmony/weakmaps.js

 // Copyright 2011 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 119 matching lines @@
 assertTrue(m instanceof WeakMap);
 
 
 // Test some common JavaScript idioms
 var m = new WeakMap;
 assertTrue(m instanceof WeakMap);
 assertTrue(WeakMap.prototype.set instanceof Function)
 assertTrue(WeakMap.prototype.get instanceof Function)
 assertTrue(WeakMap.prototype.has instanceof Function)
 assertTrue(WeakMap.prototype.delete instanceof Function)
+assertTrue(WeakMap.prototype.constructor === WeakMap)
 
 
 // Regression test for issue 1617: The prototype of the WeakMap constructor
 // needs to be unique (i.e. different from the one of the Object constructor).
 assertFalse(WeakMap.prototype === Object.prototype);
 var o = Object.create({});
 assertFalse("get" in o);
 assertFalse("set" in o);
 assertEquals(undefined, o.get);
 assertEquals(undefined, o.set);
 var o = Object.create({}, { myValue: {
   value: 10,
   enumerable: false,
   configurable: true,
   writable: true
 }});
 assertEquals(10, o.myValue);
 
 
 // Stress Test
 // There is a proposed stress-test available at the es-discuss mailing list
 // which cannot be reasonably automated.  Check it out by hand if you like:
 // https://mail.mozilla.org/pipermail/es-discuss/2011-May/014096.html