Introduce FieldIndex to unify and abstract property/field offset

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 7f3809d2580c672075d4f98a66b7af4a0eb4387d..60855a0a801f5c38d683ae442520ed046d79f83f 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -414,7 +414,8 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
         int length = properties.length();
         for (int i = 0; i < length; i++) {
           Handle<Object> value = properties[i];
-          json_object->FastPropertyAtPut(i, *value);
+          FieldIndex index = FieldIndex::ForPropertyIndex(*map, i);
+          json_object->FastPropertyAtPut(index, *value);
         }
       } else {
         key = ParseJsonInternalizedString();
@@ -438,7 +439,8 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
       int length = properties.length();
       for (int i = 0; i < length; i++) {
         Handle<Object> value = properties[i];
-        json_object->FastPropertyAtPut(i, *value);
+        FieldIndex index = FieldIndex::ForPropertyIndex(*map, i);
+        json_object->FastPropertyAtPut(index, *value);
       }
     }
   }