Introduce --no-prune-dead-locals flag to disable dead locals pruning.

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index 7caa3f90bb0f9d69673c955cdf797730e5e3874a..cdca0e44541938752e5b90441ca14e67f24a3090 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -12,6 +12,7 @@
 
 namespace dart {
 
+DEFINE_FLAG(bool, prune_dead_locals, true, "optimize dead locals away");
 DECLARE_FLAG(bool, reorder_basic_blocks);
 DECLARE_FLAG(bool, trace_optimization);
 DECLARE_FLAG(bool, verify_compiler);
@@ -872,7 +873,7 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
   // slots with null.
   BitVector* live_in = variable_liveness->GetLiveInSet(block_entry);
   for (intptr_t i = 0; i < variable_count(); i++) {
-    if (!live_in->Contains(i)) {
+    if (FLAG_prune_dead_locals && !live_in->Contains(i)) {
       (*env)[i] = constant_dead();
     }
   }
@@ -940,7 +941,8 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
           intptr_t index = store->local().BitIndexIn(num_non_copied_params_);
           result = store->value()->definition();
 
-          if (variable_liveness->IsStoreAlive(block_entry, store)) {
+          if (!FLAG_prune_dead_locals ||
+              variable_liveness->IsStoreAlive(block_entry, store)) {
             (*env)[index] = result;
           } else {
             (*env)[index] = constant_dead();
@@ -958,7 +960,8 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
             live_phis->Add(phi);
           }
 
-          if (variable_liveness->IsLastLoad(block_entry, load)) {
+          if (FLAG_prune_dead_locals &&
+              variable_liveness->IsLastLoad(block_entry, load)) {
             (*env)[index] = constant_dead();
           }