Remove Platform.upgrateCustomElements.

tests/html/utils.dart

Index: tests/html/utils.dart
diff --git a/tests/html/utils.dart b/tests/html/utils.dart
index 04c8235353f0303d9a0ce03b5971b9d6f0414c15..3d9396cab1fcfdb95086c5fbe24eeee33ba4332e 100644
--- a/tests/html/utils.dart
+++ b/tests/html/utils.dart
@@ -2,6 +2,7 @@ library TestUtils;
 
 import 'dart:async';
 import 'dart:html';
+import 'dart:js' as js;
 import 'dart:typed_data';
 import 'package:unittest/unittest.dart';
 
@@ -173,3 +174,10 @@ Future loadCustomElementPolyfill() {
 Future loadPolyfills() {
   return loadCustomElementPolyfill();
 }
+
+// Pump custom events polyfill events.
+void customElementsTakeRecords() {
+  if (js.context.hasProperty('CustomElements')) {
+    js.context['CustomElements'].callMethod('takeRecords');
+  }
+}