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

runtime/vm/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/snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 495 matching lines @@
     ASSERT(next_field_offset > 0);
     // Instance::NextFieldOffset() returns the offset of the first field in
     // a Dart object.
     bool read_as_reference = RawObject::IsCanonical(tags) ? false : true;
     intptr_t offset = Instance::NextFieldOffset();
     intptr_t result_cid = result->GetClassId();
     while (offset < next_field_offset) {
       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.
         cls_ = isolate()->class_table()->At(result_cid);
         array_ = cls_.OffsetToFieldMap();
         field_ ^= array_.At(offset >> kWordSizeLog2);
         ASSERT(!field_.IsNull());
         ASSERT(field_.Offset() == offset);
         obj_ = pobj_.raw();
         field_.RecordStore(obj_);
@@ skipping 1412 matching lines @@
       *buffer_len_ = BytesWritten();
     }
   } else {
     FreeBuffer();
     ThrowException(exception_type(), exception_msg());
   }
 }
 
 
 }  // namespace dart