Specs: move needsShadow to an annotation

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);
   }
 }
 
+// @hasShadow annotation for registering elements
+class _HasShadow {
+  const HasShadow();
+}
+const hasShadow = const _HasShadow();
+
 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
+  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
+    var shadowClass = reflectClass(hasShadow);
+    bool needsShadow = reflect(this).type.metadata.singleWhere((mirror) => mirror.type == hasShadowClass);
+    if (children != null)
+      children = children.map((node) => node is String ? new Text(node) : node).toList();
+    this._initElement(attributes, children, hostModule, needsShadow);
+  }
+  external void _initElement(Map<String, String> attributes, List children, Module hostModule, bool needsShadow);
   // 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
+  // children must be Text or Element
+  // if needsShadow is true, 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
     var tagnameClass = reflectClass(tagname);
     return (reflectClass(this.runtimeType).metadata.singleWhere((mirror) => mirror.type == tagnameClass).reflectee as tagname).value;
   }
 
   external bool hasAttribute(String name); // O(N) in number of attributes
   external String getAttribute(String name); // O(N) in number of attributes
   external void setAttribute(String name, [String value = '']); // O(N) in number of attributes
   external void removeAttribute(String name); // O(N) in number of attributes
   // 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
 
-  get bool needsShadow => false; // O(1)
   external final Root shadowRoot; // O(1)
   // returns the shadow root
 
   void endTagParsedCallback() { }
   void attributeChangeCallback(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
@@ skipping 149 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>
 ```