Provide API support for weak handles and post-mortem finalization.

runtime/vm/handles_impl.h

Index: runtime/vm/handles_impl.h
diff --git a/runtime/vm/handles_impl.h b/runtime/vm/handles_impl.h
index 3e58c959a106330f209b38e600cb43f85691b076..d8972dacb48c5f914f8cbc5da0d7e558691a5d54 100644
--- a/runtime/vm/handles_impl.h
+++ b/runtime/vm/handles_impl.h
@@ -303,7 +303,7 @@ void Handles<kHandleSizeInWords,
              kOffsetOfRawPtr>::HandlesBlock::Visit(HandleVisitor* visitor) {
   ASSERT(visitor != NULL);
   for (intptr_t i = 0; i < next_handle_slot_; i += kHandleSizeInWords) {
-    visitor->Visit(&data_[i + kOffsetOfRawPtr/kWordSize]);
+    visitor->VisitHandle(&data_[i]);
   }
 }