Don't do C++ field guards either when --use_field_guards=false.

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 7678 matching lines @@
     set_guarded_list_length(Field::kNoFixedLength);
     set_guarded_list_length_in_object_offset(Field::kUnknownLengthOffset);
   }
 
   // Expected class id or nullability of the field changed.
   return true;
 }
 
 
 void Field::RecordStore(const Object& value) const {
+  if (!FLAG_use_field_guards) {
+    return;
+  }
+
   if (FLAG_trace_field_guards) {
     OS::Print("Store %s %s <- %s\n",
               ToCString(),
               GuardedPropertiesAsCString(),
               value.ToCString());
   }
 
   if (UpdateGuardedCidAndLength(value)) {
     if (FLAG_trace_field_guards) {
       OS::Print("    => %s\n", GuardedPropertiesAsCString());
@@ skipping 13566 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart