[es6] correctly handle object wrappers in JSON.stringify.

test/mjsunit/es6/symbols.js

 // Copyright 2013 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 508 matching lines @@
   var throwFuncs = [lt, gt, le, ge, lt_same, gt_same, le_same, ge_same];
 
   for (var f of throwFuncs) {
     assertThrows(f, TypeError);
     %OptimizeFunctionOnNextCall(f);
     assertThrows(f, TypeError);
     assertThrows(f, TypeError);
   }
 }
 TestComparison();
+
+
+function TestStringify(expected, input) {
+  assertEquals(expected, JSON.stringify(input));
+  assertEquals(expected, JSON.stringify(input, null, 0));
+}
+
+TestStringify(undefined, Symbol("a"));
+var symbol_wrapper = Object(Symbol("a"))
+TestStringify('{}', symbol_wrapper);
+symbol_wrapper.a = 1;
+TestStringify('{"a":1}', symbol_wrapper);