Use the constant pool for all constants, not just null.

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 283 matching lines @@
   // Replace a target entry instruction with a join entry instruction.  Does
   // not update the original target's predecessors to point to the new block
   // and does not replace the target in already computed block order lists.
   static JoinEntryInstr* ToJoinEntry(TargetEntryInstr* target);
 
  private:
   // Match an instance of the pattern to rewrite.  See the implementation
   // for the patterns that are handled by this pass.
   static bool Match(JoinEntryInstr* block);
 
-  // Duplicate a constant, assigning it a new SSA name.
-  static ConstantInstr* CloneConstant(FlowGraph* flow_graph,
-                                      ConstantInstr* constant);
-
   // Duplicate a branch while replacing its comparison's left and right
   // inputs.
   static BranchInstr* CloneBranch(BranchInstr* branch,
                                   Value* left,
                                   Value* right);
 };
 
 
 // Rewrite diamond control flow patterns that materialize values to use more
 // efficient branchless code patterns if such are supported on the current
 // platform.
 class IfConverter : public AllStatic {
  public:
   static void Simplify(FlowGraph* flow_graph);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_