Fix MutationEvent class name

tools/dom/templates/html/impl/impl_MutationEvent.darttemplate

Index: tools/dom/templates/html/impl/impl_MutationEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_MutationEvent.darttemplate b/tools/dom/templates/html/impl/impl_MutationEvent.darttemplate
index ddb3acd498ba525ab2d63ced2fbb5183b2b4510c..550e749cbc9cc7e690e60173c4e307966fb399a2 100644
--- a/tools/dom/templates/html/impl/impl_MutationEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_MutationEvent.darttemplate
@@ -9,7 +9,7 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
       {bool canBubble: false, bool cancelable: false, Node relatedNode,
       String prevValue, String newValue, String attrName, int attrChange: 0}) {
 
-    MutationEvent event = document._createEvent('MutationEvent');
+    _MutationEvent event = document._createEvent('MutationEvent');
     event._initMutationEvent(type, canBubble, cancelable, relatedNode,
         prevValue, newValue, attrName, attrChange);
     return event;