Specs: Make appendChild, prependChild, and setChild return the child; make Dispatcher's unlisten() …

sky/specs/elements.md

 Sky DOM APIs
 ============
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 // ELEMENT TREE API
 
@@ skipping 121 matching lines @@
 
   // Returns a new List every time.
   external List<Node> getChildren(); // O(N) in number of child nodes
   List<Element> getChildElements() {
     // that the following works without a cast is absurd
     return getChildren().where((node) => node is Element).toList();
   }
 
   external void _appendChild(Node node); // O(N) in number of descendants
   // node must be Text or Element
-  void appendChild(Node node) {
+  Node appendChild(Node node) {
     if (node is String)
       node = new Text(node);
-    _appendChild(node);    
+    _appendChild(node);
+    return node;
   }
   void append(List nodes) {
     nodes.forEach(appendChild);
   }
 
   external void _prependChild(Node node); // O(N) in number of descendants
   // node must be Text or Element
-  void prependChild(Node node) {
+  Node prependChild(Node node) {
     if (node is String)
       node = new Text(node);
-    _prependChild(node);    
+    _prependChild(node);
+    return node;
   }
   void prepend(List nodes) {
     // note: not implemented in terms of _prependChild()
     if (firstChild != null)
       firstChild.insertBefore(nodes);
     else
       append(nodes);
   }
 
   external void removeChildren(); // O(N) in number of descendants
-  void setChild(Node node) {
+  Node setChild(Node node) {
     removeChildren();
     appendChild(node);
+    return node;
   }
   void setChildren(List nodes) {
     removeChildren();
     append(nodes);
   }
 }
 
 class Attr {
   const Attr (this.name, [this.value = '']); // O(1)
   final String name; // O(1)
@@ skipping 201 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>
 ```