Specs: remove previousSibling/nextSibling arguments to parentChangeCallback() since their purpose i…

sky/specs/elements.md

 Sky DOM APIs
 ============
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 // ELEMENT TREE API
 
@@ skipping 66 matching lines @@
     }
   }
 
   external void remove(); // O(N) in number of descendants
   // parentNode must be non-null
 
   // called when parentNode changes
   // this is why insertBefore(), append(), et al, are O(N) -- the whole affected subtree is walked
   // mutating the element tree from within this is strongly discouraged, since it will result in the
   // callbacks being invoked while the element tree is in a different state than implied by the callbacks
-  external void parentChangeCallback(ParentNode oldParent, ParentNode newParent, Node previousSibling, Node nextSibling); // O(N) in descendants
+  external void parentChangeCallback(ParentNode oldParent, ParentNode newParent); // O(N) in descendants
   // default implementation calls attached/detached
   void attachedCallback() { }
   void detachedCallback() { }
 
   external List<ContentElement> getDestinationInsertionPoints(); // O(N) in number of insertion points the node is in
   // returns the <content> elements to which this element was distributed
 
   external Node cloneNode({bool deep: false}); // O(1) if deep=false, O(N) in the number of descendants if deep=true
 
   external ElementStyleDeclarationList get style; // O(1)
@@ skipping 299 matching lines @@
   external bool matches(Element element); // O(F())
   external Element find(Node root); // O(N*F())+O(M) where N is the number of descendants and M the average depth of the tree
   external List<Element> findAll(Node root); // O(N*F())+O(N*M) where N is the number of descendants and M the average depth of the tree
   // find() and findAll() throw if the root is not one of the following:
   //  - Element
   //  - Fragment
   //  - Root
 }
 </script>
 ```