Treat null like the object it is in the Observatory and Service.

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.
 class RawObject;
 class Isolate;
 class ObjectPointerVisitor;
 
 // A ring buffer of object pointers that have been given an id. An object
 // may be pointed to by multiple ids. Objects contained in the ring will
 // be preserved across scavenges but not old space collections.
 // When the ring buffer wraps around older objects will be replaced and their
 // ids will be invalidated.
 class ObjectIdRing {
  public:
+  enum LookupResult {
+    kValid = 0,
+    kInvalid,    // Malformed ring id (used in service.cc).
+    kCollected,  // Entry was reclaimed due to a full GC (entries are weak).
+    kExpired,    // Entry was evicted during an insertion into a full ring.
+  };
+
   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();
 
+  // Adds the argument to the ring and returns its id. Note we do not allow
+  // adding Object::null().
   int32_t GetIdForObject(RawObject* raw_obj);
 
-  // Returns Object::sentinel() when the id is not valid.
-  RawObject* GetObjectForId(int32_t id);
+  // Returns Object::null() when the result is not kValid.
+  RawObject* GetObjectForId(int32_t id, LookupResult* kind);
 
   void VisitPointers(ObjectPointerVisitor* visitor);
 
  private:
   friend class ObjectIdRingTestHelper;
 
   void SetCapacityAndMaxSerial(int32_t capacity, int32_t max_serial);
 
   ObjectIdRing(Isolate* isolate, int32_t capacity);
   Isolate* isolate_;
@@ skipping 15 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_