Specs: pass the current <script> to the module library init()

sky/specs/frameworks.md

+Frameworks
+----------
+
+Sky is intended to support multiple frameworks. Here is one way you
+could register a custom element using Dart annotations:
+
+```dart
+// @tagname annotation for registering elements
+// only useful when placed on classes that inherit from Element
+class tagname extends AutomaticMetadata {
+  const tagname(this.name);
+  final String name;
+  void init(DeclarationMirror target, Module module, ScriptElement script) {
+    assert(target is ClassMirror);
+    if (!(target as ClassMirror).isSubclassOf(reflectClass(Element)))
+      throw new UnsupportedError('@tagname can only be used on descendants of Element');
+    module.registerElement(name, (target as ClassMirror).reflectedType);
+  }
+}
+```
+
+A framework that used the above code could use the following code to
+get the tag name of an element:
+
+```dart
+String getTagName(Element element) { // 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(element.runtimeType).metadata.singleWhere((mirror) => mirror.type == tagnameClass).reflectee as tagname).name;
+}
+```