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

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 7a800233acfb0e9099b434284e344994f9850cc6..44174e7a9f5c4992e8c6d5be50df863944504853 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -37,6 +37,7 @@ DECLARE_FLAG(bool, eliminate_type_checks);
 DECLARE_FLAG(bool, trace_optimization);
 DECLARE_FLAG(bool, trace_constant_propagation);
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
+DECLARE_FLAG(bool, prune_dead_locals);
 
 Definition::Definition(intptr_t deopt_id)
     : Instruction(deopt_id),
@@ -1067,7 +1068,11 @@ void JoinEntryInstr::InsertPhi(intptr_t var_index, intptr_t var_count) {
     }
   }
   ASSERT((*phis_)[var_index] == NULL);
-  (*phis_)[var_index] = new PhiInstr(this, PredecessorCount());
+  PhiInstr* phi = new PhiInstr(this, PredecessorCount());
+  (*phis_)[var_index] = phi;
+  if (!FLAG_prune_dead_locals) {
+    phi->mark_alive();
+  }
 }