Handle collected objects and expired handles more gracefully.

runtime/vm/object_id_ring.h

 // 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.
 
 #ifndef VM_OBJECT_ID_RING_H_
 #define VM_OBJECT_ID_RING_H_
 
 namespace dart {
 
 // Forward declarations.
@@ skipping 10 matching lines @@
  public:
   static const int32_t kMaxId = 0x3FFFFFFF;
   static const int32_t kInvalidId = -1;
   static const int32_t kDefaultCapacity = 1024;
 
   static void Init(Isolate* isolate, int32_t capacity = kDefaultCapacity);
 
   ~ObjectIdRing();
 
   int32_t GetIdForObject(RawObject* raw_obj);
+
+  // Returns Object::sentinel() when the id is not valid.
   RawObject* GetObjectForId(int32_t id);
 
   void VisitPointers(ObjectPointerVisitor* visitor);
 
  private:
   friend class ObjectIdRingTestHelper;
 
   void SetCapacityAndMaxSerial(int32_t capacity, int32_t max_serial);
 
   ObjectIdRing(Isolate* isolate, int32_t capacity);
@@ skipping 16 matching lines @@
   int32_t IndexOfId(int32_t id);
   bool IsValidContiguous(int32_t id);
   bool IsValidId(int32_t id);
 
   DISALLOW_COPY_AND_ASSIGN(ObjectIdRing);
 };
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_ID_RING_H_