Specs: remove vestiges of 'with ChildNode'

sky/specs/elements.md

 Sky DOM APIs
 ============
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 // ELEMENT TREE API
 
@@ skipping 179 matching lines @@
   const tagname(this.name);
   final String name;
   void init(DeclarationMirror target, Module module) {
     assert(target is ClassMirror);
     if (!target.isSubclassOf(reflectClass(Element)))
       throw Error('@tagname can only be used on descendants of Element');
     module.registerElement(name, (target as ClassMirror).reflectedType);
   }
 }
 
-abstract class Element extends ParentNode with Node {
+abstract class Element extends ParentNode {
   external Element({Map<String, String> attributes: null,
                    List children: null,
                    Module hostModule: null}); // O(M+N), M = number of attributes, N = number of children nodes plus all their descendants
   // initialises the internal attributes table, which is a ordered list
   // appends the given children nodes
   // children must be String, Text, or Element
   // if this.needsShadow, creates a shadow tree
 
   String get tagName { // O(N) in number of annotations on the class
     // throws a StateError if the class doesn't have an @tagname annotation
@@ skipping 22 matching lines @@
   // TODO(ianh): does a node ever need to know when it's been redistributed?
 
   @override
   Type getLayoutManager() { // O(1)
     if (renderNode)
       return renderNode.getProperty(phDisplay);
     return super.getLayoutManager();
   }
 }
 
-class Text extends Node with Node {
+class Text extends Node {
   external Text([String value = '']); // O(1)
 
   external String get value; // O(1)
   external void set (String value); // O(1)
 
   void valueChangeCallback(String oldValue, String newValue) { }
 
   @override
   Type getLayoutManager() => TextLayoutManager; // O(1)
 }
@@ skipping 131 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>
 ```