Use a trail instead of a mark bit when processing recursive types in the VM

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/bigint_operations.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 313 matching lines @@
   // Set all the object fields.
   // TODO(5411462): Need to assert No GC can happen here, even though
   // allocations may happen.
   intptr_t num_flds = (type_ref.raw()->to() - type_ref.raw()->from());
   for (intptr_t i = 0; i <= num_flds; i++) {
     (*reader->ObjectHandle()) = reader->ReadObjectRef();
     type_ref.StorePointer((type_ref.raw()->from() + i),
                           reader->ObjectHandle()->raw());
   }
 
-  type_ref.set_is_being_checked(false);
-
   return type_ref.raw();
 }
 
 
 void RawTypeRef::WriteTo(SnapshotWriter* writer,
                          intptr_t object_id,
                          Snapshot::Kind kind) {
   ASSERT(writer != NULL);
 
   // Write out the serialization header value for this object.
@@ skipping 104 matching lines @@
   // TODO(5411462): Need to assert No GC can happen here, even though
   // allocations may happen.
   intptr_t num_flds = (bounded_type.raw()->to() -
                        bounded_type.raw()->from());
   for (intptr_t i = 0; i <= num_flds; i++) {
     (*reader->ObjectHandle()) = reader->ReadObjectRef();
     bounded_type.StorePointer((bounded_type.raw()->from() + i),
                               reader->ObjectHandle()->raw());
   }
 
-  bounded_type.set_is_being_checked(false);
-
   return bounded_type.raw();
 }
 
 
 void RawBoundedType::WriteTo(SnapshotWriter* writer,
                              intptr_t object_id,
                              Snapshot::Kind kind) {
   ASSERT(writer != NULL);
 
   // Write out the serialization header value for this object.
@@ skipping 2242 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 MirrorReference)");
   } else {
     UNREACHABLE();
   }
 }
 
 }  // namespace dart