Implement Object.clone.

Also add a conditional to Thread to keep tests running.

Change-Id: I02145973af0fca823acb689e04912c2020d9c650
diff --git a/src/object.h b/src/object.h
index a04fcbb..2adee04 100644
--- a/src/object.h
+++ b/src/object.h
@@ -211,10 +211,7 @@
 
   size_t SizeOf() const;
 
-  Object* Clone() {
-    UNIMPLEMENTED(FATAL);
-    return NULL;
-  }
+  Object* Clone();
 
   static MemberOffset MonitorOffset() {
     return OFFSET_OF_OBJECT_MEMBER(Object, monitor_);