Reland "Track field types.".

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index a7dbd3ab8651d2cb9995bb994a7c1c158e4c6e7f..8c19bc847454751413cd0dfc539f6cfe991d675f 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -415,7 +415,15 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
             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;