[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 531 matching lines @@
     }
     assertTrue(false);  // should never get here.
   }
 
   assertTrue(verifier(Symbol, TypeError()));
   assertTrue(verifier(rSymbol, rError()));
   assertFalse(verifier(Symbol, rError()));
   assertFalse(verifier(rSymbol, TypeError()));
 }
 TestContext();
+
+
+function TestStringify(expected, input) {
+  assertEquals(expected, JSON.stringify(input));
+  assertEquals(expected, JSON.stringify(input, null, 0));
+}
+
+TestStringify(undefined, Symbol("a"));
+TestStringify('[{}]', [Object(Symbol())]);
+var symbol_wrapper = Object(Symbol("a"))
+TestStringify('{}', symbol_wrapper);
+symbol_wrapper.a = 1;
+TestStringify('{"a":1}', symbol_wrapper);