Create read only handles for empty_array and sentinel objects

vm/flow_graph_optimizer.h

 // 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.
 
 #ifndef VM_FLOW_GRAPH_OPTIMIZER_H_
 #define VM_FLOW_GRAPH_OPTIMIZER_H_
 
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph.h"
 
@@ skipping 165 matching lines @@
 // Sparse conditional constant propagation and unreachable code elimination.
 // Assumes that use lists are computed and preserves them.
 class ConstantPropagator : public FlowGraphVisitor {
  public:
   ConstantPropagator(FlowGraph* graph,
                      const GrowableArray<BlockEntryInstr*>& ignored);
 
   static void Optimize(FlowGraph* graph);
 
   // Used to initialize the abstract value of definitions.
-  static RawObject* Unknown() { return Object::transition_sentinel(); }
+  static RawObject* Unknown() { return Object::transition_sentinel().raw(); }
 
  private:
   void Analyze();
   void Transform();
 
   void SetReachable(BlockEntryInstr* block);
   void SetValue(Definition* definition, const Object& value);
 
   // Assign the join (least upper bound) of a pair of abstract values to the
   // first one.
@@ skipping 32 matching lines @@
 
   // Worklists of blocks and definitions.
   GrowableArray<BlockEntryInstr*> block_worklist_;
   GrowableArray<Definition*> definition_worklist_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_