Remove special case code for generalizing constants to fields.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 6c1ecbb322ea00211813ba6c9584fcbf8a227b5b..4e178b49bd8457e4a01154ac12695b865f30b9c0 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1569,7 +1569,7 @@ MaybeObject* JSObject::MigrateInstance() {
   // transition that matches the object. This achieves what is needed.
   Map* original_map = map();
   MaybeObject* maybe_result = GeneralizeFieldRepresentation(
-      0, Representation::None());
+      0, Representation::None(), ALLOW_AS_CONSTANT);
   JSObject* result;
   if (FLAG_trace_migration && maybe_result->To(&result)) {
     PrintInstanceMigration(stdout, original_map, result->map());
@@ -2361,6 +2361,7 @@ PropertyType DescriptorArray::GetType(int descriptor_number) {
 
 
 int DescriptorArray::GetFieldIndex(int descriptor_number) {
+  ASSERT(GetDetails(descriptor_number).type() == FIELD);
   return GetDetails(descriptor_number).field_index();
 }