Implement set trap for proxies, and revamp class hierarchy in preparation

test/mjsunit/json.js

Index: test/mjsunit/json.js
diff --git a/test/mjsunit/json.js b/test/mjsunit/json.js
index 812ffeb5a7ba3c74ca51f4cec34ee32945a88686..310296b64e7970d0e2892fd328f425ded9bd4c9e 100644
--- a/test/mjsunit/json.js
+++ b/test/mjsunit/json.js
@@ -382,7 +382,7 @@ var reJSON = /Is callable/;
 reJSON.toJSON = function() { return "has toJSON"; };
 
 assertEquals(
-    '[37,null,1,"foo","37","true",null,"has toJSON",null,"has toJSON"]',
+    '[37,null,1,"foo","37","true",null,"has toJSON",{},"has toJSON"]',
     JSON.stringify([num37, numFoo, numTrue,
                     strFoo, str37, strTrue,
                     func, funcJSON, re, reJSON]));