Fixing another case of multiple upgrades of a custom element

Source/bindings/dart/DartDOMWrapper.h

Index: Source/bindings/dart/DartDOMWrapper.h
diff --git a/Source/bindings/dart/DartDOMWrapper.h b/Source/bindings/dart/DartDOMWrapper.h
index ce7b9dd0877ad4ffd0c6cca14afc54deffe9326e..e6503db9a47164af933b928765823bdd5c41d336 100644
--- a/Source/bindings/dart/DartDOMWrapper.h
+++ b/Source/bindings/dart/DartDOMWrapper.h
@@ -272,11 +272,18 @@ private:
     }
 
     template <class BindingsClass>
-    static void wrapperWeakCallback(Dart_WeakPersistentHandle, void* peer)
+    static void wrapperWeakCallback(Dart_WeakPersistentHandle wrapper, void* peer)
     {
         typedef DartDOMWrapperTraits<BindingsClass> Traits;
         DartDOMData* domData = DartDOMData::current();
         typename BindingsClass::NativeType* domObject = static_cast<typename BindingsClass::NativeType*>(peer);
+        Dart_WeakPersistentHandle currentWrapper = Traits::MapTraits::domMap(domData)->get(domObject);

[vsm, 2013/10/31 23:19:43]

Looking the IDB failures - if they are from this - you'll need a
DartIsolate/ApiScope:

../../dart/runtime/vm/handles_impl.h:99: error: expected:
isolate->no_handle_scope_depth() == 0

+        // This could be an old wrapper which has been replaced with a custom element.
+        if (currentWrapper != wrapper) {
+            ASSERT(!Dart_IdentityEquals(Dart_HandleFromWeakPersistent(currentWrapper), Dart_HandleFromWeakPersistent(wrapper)));
+            return;
+        }
+
         if (DartDOMWrapperTraits<BindingsClass>::MapTraits::domMap(domData)->contains(domObject)) {
             Traits::MapTraits::domMap(domData)->remove(domObject);
             Traits::ActiveTraits::removeWrapper(domData, domObject);