Do not add smis to the store buffer when cloning objects.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 17755)
+++ runtime/vm/raw_object.h	(working copy)
@@ -253,10 +253,12 @@
   }
 
   bool IsNewObject() const {
+    ASSERT(IsHeapObject());
     uword addr = reinterpret_cast<uword>(this);
     return (addr & kNewObjectAlignmentOffset) == kNewObjectAlignmentOffset;
   }
   bool IsOldObject() const {
+    ASSERT(IsHeapObject());
     uword addr = reinterpret_cast<uword>(this);
     return (addr & kNewObjectAlignmentOffset) == kOldObjectAlignmentOffset;
   }