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

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 746e2ab11152ef5f6e332d3d74d608ea15b69093..5eddb352d215ad7181971cce4d986141d86fd26f 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -513,7 +513,7 @@ RawObject* SnapshotReader::ReadInstance(intptr_t object_id,
       pobj_ = ReadObjectImpl(read_as_reference);
       result->SetFieldAtOffset(offset, pobj_);
       if ((offset != type_argument_field_offset) &&
-          (kind_ == Snapshot::kMessage) && FLAG_use_field_guards) {
+          (kind_ == Snapshot::kMessage) && isolate()->use_field_guards()) {
         // TODO(fschneider): Consider hoisting these lookups out of the loop.
         // This would involve creating a handle, since cls_ can't be reused
         // across the call to ReadObjectImpl.