Add mutable double boxes for fields.

runtime/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 41 matching lines @@
                                  intptr_t token_pos,
                                  const ICData& ic_data,
                                  TargetEntryInstr** entry,
                                  Definition** last);
 
   // Remove environments from the instructions which do not deoptimize.
   void EliminateEnvironments();
 
   virtual void VisitStaticCall(StaticCallInstr* instr);
   virtual void VisitInstanceCall(InstanceCallInstr* instr);
+  virtual void VisitStoreInstanceField(StoreInstanceFieldInstr* instr);
 
   void InsertBefore(Instruction* next,
                     Instruction* instr,
                     Environment* env,
                     Definition::UseKind use_kind) {
     flow_graph_->InsertBefore(next, instr, env, use_kind);
   }
 
  private:
   // Attempt to build ICData for call using propagated class-ids.
@@ skipping 314 matching lines @@
 // Optimize spill stores inside try-blocks by identifying values that always
 // contain a single known constant at catch block entry.
 class TryCatchAnalyzer : public AllStatic {
  public:
   static void Optimize(FlowGraph* flow_graph);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_