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

runtime/vm/raw_object_snapshot.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/native_entry.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
@@ skipping 854 matching lines @@
   field.set_guarded_cid(reader->Read<int32_t>());
   field.set_is_nullable(reader->Read<int32_t>());
   field.set_kind_bits(reader->Read<uint8_t>());
   field.set_kernel_field(NULL);
 
   // Set all the object fields.
   READ_OBJECT_FIELDS(field, field.raw()->from(), field.raw()->to_snapshot(kind),
                      kAsReference);
   field.StorePointer(&field.raw_ptr()->dependent_code_, Array::null());
 
-  if (!FLAG_use_field_guards) {
+  if (!reader->isolate()->use_field_guards()) {
     field.set_guarded_cid(kDynamicCid);
     field.set_is_nullable(true);
     field.set_guarded_list_length(Field::kNoFixedLength);
     field.set_guarded_list_length_in_object_offset(Field::kUnknownLengthOffset);
   } else {
     field.InitializeGuardedListLengthInObjectOffset();
   }
 
   return field.raw();
 }
@@ skipping 2182 matching lines @@
     // We do not allow objects with native fields in an isolate message.
     writer->SetWriteException(Exceptions::kArgument,
                               "Illegal argument in isolate message"
                               " : (object is a UserTag)");
   } else {
     UNREACHABLE();
   }
 }
 
 }  // namespace dart