Environment sets the environment position when copying to maintain the use lists.

runtime/vm/flow_graph_optimizer.cc

 // 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.
 
 #include "vm/flow_graph_optimizer.h"
 
 #include "vm/cha.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/hash_map.h"
 #include "vm/il_printer.h"
@@ skipping 234 matching lines @@
       return false;
   }
 }
 
 
 BindInstr* FlowGraphOptimizer::InsertBefore(Instruction* instr,
                                             Computation* comp,
                                             Environment* env,
                                             BindInstr::UseKind use_kind) {
   BindInstr* bind = new BindInstr(use_kind, comp);
-  if (env != NULL) bind->set_env(env->Copy());
+  if (env != NULL) env->CopyTo(bind);
   if (use_kind == BindInstr::kUsed) {
     bind->set_ssa_temp_index(flow_graph_->alloc_ssa_temp_index());
   }
   bind->InsertBefore(instr);
   return bind;
 }
 
 
 BindInstr* FlowGraphOptimizer::InsertAfter(Instruction* instr,
                                            Computation* comp,
                                            Environment* env,
                                            BindInstr::UseKind use_kind) {
   BindInstr* bind = new BindInstr(use_kind, comp);
-  if (env != NULL) bind->set_env(env->Copy());
+  if (env != NULL) env->CopyTo(bind);
   if (use_kind == BindInstr::kUsed) {
     bind->set_ssa_temp_index(flow_graph_->alloc_ssa_temp_index());
   }
   bind->InsertAfter(instr);
   return bind;
 }
 
 
 bool FlowGraphOptimizer::TryReplaceWithBinaryOp(BindInstr* instr,
                                                 InstanceCallComp* comp,
@@ skipping 769 matching lines @@
       DirectChainedHashMap<BindInstr*> child_map(*map);  // Copy map.
       OptimizeRecursive(child, &child_map);
     } else {
       OptimizeRecursive(child, map);  // Reuse map for the last child.
     }
   }
 }
 
 
 }  // namespace dart