Specs: rename sky:core to dart:sky

sky/specs/elements.md

 Sky DOM APIs
 ============
 
 ```dart
 SKY MODULE
-<!-- part of sky:core -->
+<!-- part of dart:sky -->
 
 <script>
 // ELEMENT TREE API
 
 abstract class Node extends EventTarget {
   @override
   external List<EventTarget> getEventDispatchChain(); // O(N) in number of ancestors across shadow trees
   // implements EventTarget.getEventDispatchChain()
   // returns the event dispatch chain (including handling shadow trees)
 
@@ skipping 364 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>
 ```