[json] use OrderedHashSet for replacer property list.

test/mjsunit/json.js

Index: test/mjsunit/json.js
diff --git a/test/mjsunit/json.js b/test/mjsunit/json.js
index 3652febc3c41d8b9c7fb217fdafb8c0c53a8c345..102f777c743f6dc23d8fbb93f9c705c23e2f9480 100644
--- a/test/mjsunit/json.js
+++ b/test/mjsunit/json.js
@@ -327,6 +327,7 @@ assertEquals('{"x":5}', JSON.stringify({x:5,y:6}, ['x']));
 assertEquals('{\n "a": "b",\n "c": "d"\n}',
              JSON.stringify({a:"b",c:"d"}, null, 1));
 assertEquals('{"y":6,"x":5}', JSON.stringify({x:5,y:6}, ['y', 'x']));
+assertEquals('{"y":6,"x":5}', JSON.stringify({x:5,y:6}, ['y', 'x', 'x', 'y']));
 
 // toJSON get string keys.
 var checker = {};