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

runtime/vm/object_id_ring.cc

Index: runtime/vm/object_id_ring.cc
diff --git a/runtime/vm/object_id_ring.cc b/runtime/vm/object_id_ring.cc
index fd069888cd6b2ea42c99f300b9aaf946180ada88..a3fe46c31b6cf8d477dcfb2890663e6c92fbd3fb 100644
--- a/runtime/vm/object_id_ring.cc
+++ b/runtime/vm/object_id_ring.cc
@@ -26,18 +26,25 @@ ObjectIdRing::~ObjectIdRing() {
 
 
 int32_t ObjectIdRing::GetIdForObject(RawObject* object) {
+  // We do not allow inserting null or any of the psuedo-nulls because the are
+  // used as sentinel values for expired and reclaimed entries.
+  ASSERT(!object->IsNullOrPseudoNull());

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

Technically we could allow null to be inserted now that we have LookupResult...
but it's still better not to let it in.

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

No, the regular null is how we know an entry was collected because the GC places
a regular null.

Narrowed this to disallowing only regular null based on returning regular null
for !kValid lookups.

   return AllocateNewId(object);
 }
 
 
-RawObject* ObjectIdRing::GetObjectForId(int32_t id) {
+RawObject* ObjectIdRing::GetObjectForId(int32_t id, LookupResult* kind) {
+  *kind = kValid;
   int32_t index = IndexOfId(id);
   if (index == kInvalidId) {
-    // Return sentinel to allow caller to distinguish expired ids.
-    return Object::sentinel().raw();
+    *kind = kExpired;
+    return Object::null();
   }
   ASSERT(index >= 0);
   ASSERT(index < capacity_);
+  if (table_[index] == Object::null()) {
+    *kind = kCollected;
+  }
   return table_[index];
 }