Improve load forwarding:

runtime/vm/flow_graph_inliner.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_INLINER_H_
 #define VM_FLOW_GRAPH_INLINER_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
 
 class Field;
 class FlowGraph;
 template <typename T> class GrowableArray;
 
 class FlowGraphInliner : ValueObject {
  public:
-  FlowGraphInliner(FlowGraph* flow_graph, GrowableArray<Field*>* guarded_fields)
+  FlowGraphInliner(FlowGraph* flow_graph,
+                   GrowableArray<const Field*>* guarded_fields)
       : flow_graph_(flow_graph), guarded_fields_(guarded_fields) {}
 
   // The flow graph is destructively updated upon inlining.
   void Inline();
 
   static void CollectGraphInfo(FlowGraph* flow_graph);
 
  private:
   FlowGraph* flow_graph_;
-  GrowableArray<Field*>* guarded_fields_;
+  GrowableArray<const Field*>* guarded_fields_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphInliner);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_INLINER_H_