add @CustomElement

lib/src/custom_element.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+library web_components.custom_element;
+
+import 'dart:html' show document;
+import 'package:initialize/initialize.dart';
+
+/// Annotation which registers a custom element with the main document.
+class CustomElement implements Initializer<Type> {
+  /// The tag you want to register the class to handle.
+  final String tag;
+
+  /// If this element extends a native html element, then this is the tag that
+  /// represents that element.
+  final String extendsTag;
+
+  const CustomElement(this.tag, {this.extendsTag});
+
+  void initialize(Type t) =>
+      document.registerElement(tag, t, extendsTag: extendsTag);
+}