Track field types.

src/json-parser.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 397 matching lines @@
           PropertyDetails details =
               target->instance_descriptors()->GetDetails(descriptor);
           Representation expected_representation = details.representation();
 
           if (value->FitsRepresentation(expected_representation)) {
             // If the target representation is double and the value is already
             // double, use the existing box.
             if (value->IsSmi() && expected_representation.IsDouble()) {
               value = factory()->NewHeapNumber(
                   Handle<Smi>::cast(value)->value());
+            } else if (expected_representation.IsHeapObject() &&
+                       !target->instance_descriptors()->GetFieldType(
+                           descriptor)->NowContains(value)) {
+              Handle<HeapType> value_type(value->OptimalType(
+                      isolate(), expected_representation));
+              Map::GeneralizeFieldType(target, descriptor, value_type);
             }
+            ASSERT(target->instance_descriptors()->GetFieldType(
+                    descriptor)->NowContains(value));
             properties.Add(value, zone());
             map = target;
             continue;
           } else {
             transitioning = false;
           }
         }
 
         // Commit the intermediate state to the object and stop transitioning.
         JSObject::AllocateStorageForMap(json_object, map);
@@ skipping 367 matching lines @@
 
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_