introduce web_components pkg for consolidated polyfills

tests/html/custom/created_callback_test.dart

Index: tests/html/custom/created_callback_test.dart
diff --git a/tests/html/custom/created_callback_test.dart b/tests/html/custom/created_callback_test.dart
index 49845c154e33178e52085fdbd818f9d1df4495f2..865252003d3de43db8ada71eaa96c770de59bc7e 100644
--- a/tests/html/custom/created_callback_test.dart
+++ b/tests/html/custom/created_callback_test.dart
@@ -62,7 +62,7 @@ main() {
 
   var registered = false;
   setUp(() {
-    return loadPolyfills().then((_) {
+    return customElementsReady.then((_) {
       if (!registered) {
         registered = true;
         document.register(B.tag, B);