Fix JSON.stringify wrt harmony proxies.

test/mjsunit/json2.js

 // Copyright 2012 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 91 matching lines @@
 tojson_obj = { toJSON: function(key) { return key + key; } };
 var tojson_with_key_1 = { a: tojson_obj, b: tojson_obj };
 assertEquals('{"a":"aa","b":"bb"}', JSON.stringify(tojson_with_key_1));
 var tojson_with_key_2 = [ tojson_obj, tojson_obj ];
 assertEquals('["00","11"]', JSON.stringify(tojson_with_key_2));
 
 // Test toJSON with exception.
 var tojson_ex = { toJSON: function(key) { throw "123" } };
 assertThrows(function() { JSON.stringify(tojson_ex); });
 
+// Test toJSON with access to this.
+var obj = { toJSON: function(key) { return this.a + key; }, a: "x" };
+assertEquals('{"y":"xy"}', JSON.stringify({y: obj}));
+
 // Test holes in arrays.
 var fast_smi = [1, 2, 3, 4];
 fast_smi.__proto__ = [7, 7, 7, 7];
 delete fast_smi[2];
 assertTrue(%HasFastSmiElements(fast_smi));
 assertEquals("[1,2,7,4]", JSON.stringify(fast_smi));
 
 var fast_double = [1.1, 2, 3, 4];
 fast_double.__proto__ = [7, 7, 7, 7];
 
 delete fast_double[2];
 assertTrue(%HasFastDoubleElements(fast_double));
 assertEquals("[1.1,2,7,4]", JSON.stringify(fast_double));
 
 var fast_obj = [1, 2, {}, {}];
 fast_obj.__proto__ = [7, 7, 7, 7];
 
 delete fast_obj[2];
 assertTrue(%HasFastObjectElements(fast_obj));
 assertEquals("[1,2,7,{}]", JSON.stringify(fast_obj));