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

test/mjsunit/harmony/simd.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-simd --harmony-tostring  --harmony-reflect
 // Flags: --allow-natives-syntax --expose-natives-as natives --noalways-opt
 
 function lanesForType(typeName) {
   // The lane count follows the first 'x' in the type name, which begins with
   // 'float', 'int', or 'bool'.
@@ skipping 600 matching lines @@
   assertSame(Object.getPrototypeOf(SIMD), Object.prototype);
   assertSame(SIMD + "", "[object SIMD]");
   // The SIMD object is mutable.
   SIMD.foo = "foo";
   assertSame(SIMD.foo, "foo");
   delete SIMD.foo;
   delete SIMD.Bool8x16;
   assertSame(SIMD.Bool8x16, undefined);
 }
 TestSIMDObject()
+
+
+function TestStringify(expected, input) {
+  assertEquals(expected, JSON.stringify(input));
+  assertEquals(expected, JSON.stringify(input, null, 0));
+}
+
+TestStringify(undefined, SIMD.Float32x4(1, 2, 3, 4));
+TestStringify('[null]', [SIMD.Float32x4(1, 2, 3, 4)]);
+TestStringify('[{}]', [Object(SIMD.Float32x4(1, 2, 3, 4))]);
+var simd_wrapper = Object(SIMD.Float32x4(1, 2, 3, 4));
+TestStringify('{}', simd_wrapper);
+simd_wrapper.a = 1;
+TestStringify('{"a":1}', simd_wrapper);