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

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 17755)
+++ runtime/vm/object.cc	(working copy)
@@ -1300,14 +1300,15 @@
 }
 
 
-class StoreBufferObjectPointerVisitor : public ObjectPointerVisitor {
+class StoreBufferUpdateVisitor : public ObjectPointerVisitor {
  public:
-  explicit StoreBufferObjectPointerVisitor(Isolate* isolate) :
-      ObjectPointerVisitor(isolate) {
-  }
+  explicit StoreBufferUpdateVisitor(Isolate* isolate) :
+      ObjectPointerVisitor(isolate) { }
+
   void VisitPointers(RawObject** first, RawObject** last) {
     for (RawObject** curr = first; curr <= last; ++curr) {
-      if ((*curr)->IsNewObject()) {
+      RawObject* raw_obj = *curr;
+      if (raw_obj->IsHeapObject() && raw_obj->IsNewObject()) {
         uword ptr = reinterpret_cast<uword>(curr);
         isolate()->store_buffer()->AddPointer(ptr);
       }
@@ -1315,7 +1316,7 @@
   }
 
  private:
-  DISALLOW_COPY_AND_ASSIGN(StoreBufferObjectPointerVisitor);
+  DISALLOW_COPY_AND_ASSIGN(StoreBufferUpdateVisitor);
 };
 
 
@@ -1337,7 +1338,7 @@
   NoGCScope no_gc;
   memmove(raw_obj->ptr(), src.raw()->ptr(), size);
   if (space == Heap::kOld) {
-    StoreBufferObjectPointerVisitor visitor(Isolate::Current());
+    StoreBufferUpdateVisitor visitor(Isolate::Current());
     raw_obj->VisitPointers(&visitor);
   }
   return raw_obj;