Specs: make the change callbacks all be consistently past tense ('changed')

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); // O(N) in descendants
+  external void parentChangedCallback(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 137 matching lines @@
   // calling setAttribute() with a null value removes the attribute
   // (calling it without a value sets it to the empty string)
 
   // Returns a new Array and new Attr instances every time.
   external List<Attr> getAttributes(); // O(N) in number of attributes
 
   external Root get shadowRoot; // O(1)
   // returns the shadow root
 
   void endTagParsedCallback() { }
-  void attributeChangeCallback(String name, String oldValue, String newValue) { }
+  void attributeChangedCallback(String name, String oldValue, String newValue) { }
   // name will never be null when this is called by sky
 
   // 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 {
   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) { }
+  void valueChangedCallback(String oldValue, String newValue) { }
 
   @override
   Type getLayoutManager() => TextLayoutManager; // O(1)
 }
 
 class Fragment extends ParentNode {
   Fragment({List children}); // O(N) in number of arguments plus all their descendants
   // children must be String, Text, or Element
 }
 
@@ skipping 130 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>
 ```