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

runtime/vm/clustered_snapshot.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 "vm/clustered_snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 1075 matching lines @@
       }
       field->ptr()->kind_bits_ = d->Read<uint8_t>();
     }
   }
 
   void PostLoad(const Array& refs, Snapshot::Kind kind, Zone* zone) {
     NOT_IN_PRODUCT(TimelineDurationScope tds(
         Thread::Current(), Timeline::GetIsolateStream(), "PostLoadField"));
 
     Field& field = Field::Handle(zone);
-    if (!FLAG_use_field_guards) {
+    if (!Isolate::Current()->use_field_guards()) {
       for (intptr_t i = start_index_; i < stop_index_; i++) {
         field ^= refs.At(i);
         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 {
       for (intptr_t i = start_index_; i < stop_index_; i++) {
@@ skipping 4407 matching lines @@
     thread_->isolate()->SetupImagePage(data_buffer_,
                                        /* is_executable */ false);
   }
 
   deserializer.ReadIsolateSnapshot(thread_->isolate()->object_store());
 
   return ApiError::null();
 }
 
 }  // namespace dart