VM: Allow configuring use_field_guards on the per-isolate basis and include it into snapshot featur…

runtime/vm/kernel_to_il.cc

 // Copyright (c) 2016, 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 <map>
 #include <set>
 #include <string>
 
 #include "vm/kernel_to_il.h"
 
@@ skipping 2670 matching lines @@
 
   return instructions;
 }
 
 
 Fragment FlowGraphBuilder::StoreInstanceFieldGuarded(
     const dart::Field& field,
     bool is_initialization_store) {
   Fragment instructions;
   const dart::Field& field_clone = MayCloneField(Z, field);
-  if (FLAG_use_field_guards) {
+  if (I->use_field_guards()) {
     LocalVariable* store_expression = MakeTemporary();
     instructions += LoadLocal(store_expression);
     instructions += GuardFieldClass(field_clone, H.thread()->GetNextDeoptId());
     instructions += LoadLocal(store_expression);
     instructions += GuardFieldLength(field_clone, H.thread()->GetNextDeoptId());
   }
   instructions += StoreInstanceField(field_clone, is_initialization_store);
   return instructions;
 }
 
@@ skipping 3682 matching lines @@
     thread->clear_sticky_error();
     return error.raw();
   }
 }
 
 
 }  // namespace kernel
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)