Handle collected objects and expired handles more gracefully.

runtime/vm/object_id_ring.cc

 // Copyright (c) 2013, 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 "platform/assert.h"
 #include "vm/dart_api_state.h"
 #include "vm/object_id_ring.h"
 
 namespace dart {
 
@@ skipping 15 matching lines @@
 
 
 int32_t ObjectIdRing::GetIdForObject(RawObject* object) {
   return AllocateNewId(object);
 }
 
 
 RawObject* ObjectIdRing::GetObjectForId(int32_t id) {
   int32_t index = IndexOfId(id);
   if (index == kInvalidId) {
-    return Object::null();
+    // Return sentinel to allow caller to distinguish expired ids.
+    return Object::sentinel().raw();
   }
   ASSERT(index >= 0);
   ASSERT(index < capacity_);
   return table_[index];
 }
 
 
 void ObjectIdRing::VisitPointers(ObjectPointerVisitor* visitor) {
   ASSERT(table_ != NULL);
   visitor->VisitPointers(table_, capacity_);
@@ skipping 111 matching lines @@
       const int32_t max_serial_num = max_serial_;
       const int32_t bottom = max_serial_num - (capacity_ - serial_num_);
       return id >= bottom && bottom < max_serial_num;
     }
   }
   ASSERT(wrapped_ == false);
   return IsValidContiguous(id);
 }
 
 }  // namespace dart