Implement simple dead store elimination.

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 270 matching lines @@
   static bool Optimize(FlowGraph* graph);
 
  private:
   static bool OptimizeRecursive(
       FlowGraph* graph,
       BlockEntryInstr* entry,
       CSEInstructionMap* map);
 };
 
 
+class DeadStoreElimination : public AllStatic {
+ public:
+  static void Optimize(FlowGraph* graph);
+};
+
+
 // 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);
 
   // (1) Visit branches to optimize away unreachable blocks discovered  by range
@@ skipping 128 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_