[dartium] Use ScriptWrappable when there are multiple Dart wrappers.

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 b2da8f71b1e3a70098d8f205eef3b96f0ed67299..9c3fff830dbe1ef7a4bc21d424c771ce110563c4 100644
--- a/Source/bindings/dart/DartDOMWrapper.h
+++ b/Source/bindings/dart/DartDOMWrapper.h
@@ -65,9 +65,8 @@ public:
         ASSERT(domData);
         if (ScriptWrappable::wrapperCanBeStoredInObject(domObject)) {
             Dart_WeakPersistentHandle wrapper = (Dart_WeakPersistentHandle)(ScriptWrappable::fromObject(domObject)->getDartWrapper(domData));
-            if (wrapper)
-                return wrapper;
-            // FIXMEDART: Assert absence from map when multi-isolate wrapper info is implemeted.
+            ASSERT(wrapper == Traits::MapTraits::domMap(domData)->get(domObject));
+            return wrapper;
         }
         return Traits::MapTraits::domMap(domData)->get(domObject);
     }
@@ -301,10 +300,11 @@ private:
         Dart_WeakPersistentHandle currentWrapper = 0;
         if (ScriptWrappable::wrapperCanBeStoredInObject(domObject)) {
             currentWrapper = (Dart_WeakPersistentHandle)(ScriptWrappable::fromObject(domObject)->getDartWrapper(domData));
-        }
-        if (!currentWrapper) {
+            ASSERT(currentWrapper == Traits::MapTraits::domMap(domData)->get(domObject));
+        } else {
             currentWrapper = Traits::MapTraits::domMap(domData)->get(domObject);
         }
+
         // This could be an old wrapper which has been replaced with a custom element.
         if (currentWrapper != wrapper) {
 #ifdef DEBUG