When a value has been optimized away by the compiler, set its value to

runtime/vm/flow_graph.h

 // Copyright (c) 2013, 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_H_
 #define VM_FLOW_GRAPH_H_
 
 #include "vm/growable_array.h"
 #include "vm/intermediate_language.h"
 #include "vm/parser.h"
@@ skipping 96 matching lines @@
   intptr_t allocate_block_id() { return ++max_block_id_; }
 
   GraphEntryInstr* graph_entry() const {
     return graph_entry_;
   }
 
   ConstantInstr* constant_null() const {
     return constant_null_;
   }
 
+  ConstantInstr* constant_dead() const {
+    return constant_dead_;
+  }
+
   intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
 
   intptr_t InstructionCount() const;
 
   ConstantInstr* GetConstant(const Object& object);
   void AddToInitialDefinitions(Definition* defn);
 
   void InsertBefore(Instruction* next,
                     Instruction* instr,
                     Environment* env,
@@ skipping 135 matching lines @@
   const ParsedFunction& parsed_function_;
   const intptr_t num_copied_params_;
   const intptr_t num_non_copied_params_;
   const intptr_t num_stack_locals_;
   GraphEntryInstr* graph_entry_;
   GrowableArray<BlockEntryInstr*> preorder_;
   GrowableArray<BlockEntryInstr*> postorder_;
   GrowableArray<BlockEntryInstr*> reverse_postorder_;
   GrowableArray<BlockEntryInstr*> optimized_block_order_;
   ConstantInstr* constant_null_;
+  ConstantInstr* constant_dead_;
 
   BlockEffects* block_effects_;
   bool licm_allowed_;
 
   bool use_far_branches_;
 
   ZoneGrowableArray<BlockEntryInstr*>* loop_headers_;
   ZoneGrowableArray<BitVector*>* loop_invariant_loads_;
   ZoneGrowableArray<const Field*>* guarded_fields_;
 };
@@ skipping 94 matching lines @@
   // Per block sets of available blocks. Block A is available at the block B if
   // and only if A dominates B and all paths from A to B are free of side
   // effects.
   GrowableArray<BitVector*> available_at_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_H_