Create read only handles for empty_array and sentinel objects

vm/flow_graph_optimizer.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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/flow_graph_compiler.h"
@@ skipping 3494 matching lines @@
   }
   return changed;
 }
 
 
 ConstantPropagator::ConstantPropagator(
     FlowGraph* graph,
     const GrowableArray<BlockEntryInstr*>& ignored)
     : FlowGraphVisitor(ignored),
       graph_(graph),
-      unknown_(Object::ZoneHandle(Object::transition_sentinel())),
-      non_constant_(Object::ZoneHandle(Object::sentinel())),
+      unknown_(Object::transition_sentinel()),
+      non_constant_(Object::sentinel()),
       reachable_(new BitVector(graph->preorder().length())),
       definition_marks_(new BitVector(graph->max_virtual_register_number())),
       block_worklist_(),
       definition_worklist_() {}
 
 
 void ConstantPropagator::Optimize(FlowGraph* graph) {
   GrowableArray<BlockEntryInstr*> ignored;
   ConstantPropagator cp(graph, ignored);
   cp.Analyze();
@@ skipping 824 matching lines @@
 
   if (FLAG_trace_constant_propagation) {
     OS::Print("\n==== After constant propagation ====\n");
     FlowGraphPrinter printer(*graph_);
     printer.PrintBlocks();
   }
 }
 
 
 }  // namespace dart