add @CustomElement

lib/web_components.dart

Index: lib/web_components.dart
diff --git a/lib/web_components.dart b/lib/web_components.dart
new file mode 100644
index 0000000000000000000000000000000000000000..379d04e689ba1d237e771f21872288f48e43fdc3
--- /dev/null
+++ b/lib/web_components.dart
@@ -0,0 +1,8 @@
+// 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;
+
+export 'src/custom_element.dart';
+export 'custom_element_proxy.dart';
+export 'html_import_annotation.dart';