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

test/mjsunit/es6/symbols.js

Index: test/mjsunit/es6/symbols.js
diff --git a/test/mjsunit/es6/symbols.js b/test/mjsunit/es6/symbols.js
index 58142cf27f04be74707acda6840a2ca8f9305eea..85c1358456b1fb398f2fdca5151c4d6235a7745c 100644
--- a/test/mjsunit/es6/symbols.js
+++ b/test/mjsunit/es6/symbols.js
@@ -526,3 +526,15 @@ function TestComparison() {
   }
 }
 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);