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

runtime/vm/object_id_ring.h

Index: runtime/vm/object_id_ring.h
diff --git a/runtime/vm/object_id_ring.h b/runtime/vm/object_id_ring.h
index ee9038d3fe56c59a432a06992e6a844e1a18bd8f..723bc72276e75e3c740270c8a48e7c8feb1e6e3f 100644
--- a/runtime/vm/object_id_ring.h
+++ b/runtime/vm/object_id_ring.h
@@ -19,6 +19,13 @@ class ObjectPointerVisitor;
 // ids will be invalidated.
 class ObjectIdRing {
  public:
+  enum LookupResult {

[rmacnak, 2014/08/26 21:36:50]

Not thrilled about this name.

[turnidge, 2014/08/27 16:13:56]

Sure.  I'm having trouble coming up with an improvement though.  Maybe if
GetObjectForId were called "Lookup" then the type name would feel better.  Not
necessarily suggesting that.

Maybe add one-line comments for kInvalid, kCollected, and kExpired.

[rmacnak, 2014/08/27 18:09:59]

Added one-liners.

+    kValid = 0,
+    kInvalid,
+    kCollected,
+    kExpired,
+  };
+
   static const int32_t kMaxId = 0x3FFFFFFF;
   static const int32_t kInvalidId = -1;
   static const int32_t kDefaultCapacity = 1024;
@@ -30,7 +37,7 @@ class ObjectIdRing {
   int32_t GetIdForObject(RawObject* raw_obj);
 
   // Returns Object::sentinel() when the id is not valid.

[turnidge, 2014/08/27 16:13:56]

This comment is now out of date with the code... but I'm wondering if we should
change the code to return Object::sentinel() whenever the LookupResult is not
kValid?  Or something like that -- always the same result in those three cases.

[rmacnak, 2014/08/27 18:09:59]

Updated to say we return Object::null() when !kValid.

-  RawObject* GetObjectForId(int32_t id);
+  RawObject* GetObjectForId(int32_t id, LookupResult* kind);
 
   void VisitPointers(ObjectPointerVisitor* visitor);