Fix JSON.parse typo which causes the input not to be string converted.

test/mjsunit/json.js

 // Copyright 2009 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 67 matching lines @@
 n3.toISOString = function () { return {}; };
 assertThrows(function () { n3.toJSON(); }, TypeError, "result_not_primitive");
 var n4 = new Date(10003);
 n4.toISOString = function () {
   assertEquals(0, arguments.length);
   assertEquals(this, n4);
   return null;
 };
 assertEquals(null, n4.toJSON());
 
-assertEquals(Object.prototype, JSON.__proto__);
+assertTrue(Object.prototype === JSON.__proto__);
 assertEquals("[object JSON]", Object.prototype.toString.call(JSON));
 
 // DontEnum
 for (var p in this)
   assertFalse(p == "JSON");
 
 // Parse
 assertEquals({}, JSON.parse("{}"));
 assertEquals({42:37}, JSON.parse('{"42":37}'));
 assertEquals(null, JSON.parse("null"));
@@ skipping 207 matching lines @@
              JSON.stringify({a: undefined, b: function(){}, c: 42, d: 42},
                             function(k, v) { if (k == "c") return undefined; 
                                              if (k == "d") return function(){};
                                              return v; }));
 
 TestInvalid('1); throw "foo"; (1');
 
 var x = 0;
 eval("(1); x++; (1)");
 TestInvalid('1); x++; (1');
+
+// Test string conversion of argument.
+var o = { toString: function() { return "42"; } };
+assertEquals(42, JSON.parse(o));