Create read only handles for empty_array and sentinel objects

vm/code_descriptors.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/code_descriptors.h"
 
 namespace dart {
 
 void DescriptorList::AddDescriptor(PcDescriptors::Kind kind,
                                    intptr_t pc_offset,
@@ skipping 47 matching lines @@
     }
   }
   return true;
 }
 
 
 RawArray* StackmapTableBuilder::FinalizeStackmaps(const Code& code) {
   ASSERT(Verify());
   intptr_t num_entries = Length();
   if (num_entries == 0) {
-    return Object::empty_array();
+    return Object::empty_array().raw();
   }
   uword entry_point = code.EntryPoint();
   for (intptr_t i = 0; i < num_entries; i++) {
     stack_map_ = MapAt(i);
     stack_map_.SetPC(entry_point + stack_map_.PC());
     stack_map_.SetCode(code);
   }
   return Array::MakeArray(list_);
 }
 
 
 RawStackmap* StackmapTableBuilder::MapAt(int index) const {
   Stackmap& map = Stackmap::Handle();
   map ^= list_.At(index);
   return map.raw();
 }
 
 }  // namespace dart