Reapply remove upgradeCustomElement methods.

tests/html/custom/created_callback_test.dart

 // Copyright (c) 2013, 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 created_callback_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import 'dart:js' as js;
 import '../utils.dart';
@@ skipping 70 matching lines @@
   test('unresolved and created callback timing', () {
     var div = new DivElement();
     C.div = div;
     div.setInnerHtml("""
 <x-c id="t"></x-c>
 <x-b id="u"></x-b>
 <x-c id="v"></x-c>
 <x-b id="w"></x-b>
 """, treeSanitizer: new NullTreeSanitizer());
 
-    Platform.upgradeCustomElements(div);
+    upgradeCustomElements(div);
 
     expect(C.createdInvocations, 2);
     expect(div.query('#w') is B, isTrue);
   });
 
   test('nesting of constructors', NestedElement.test);
 
   test('access while upgrading gets unupgraded element',
       AccessWhileUpgradingElement.test);
 
@@ skipping 178 matching lines @@
   } catch(e) {
     rethrow;
   } finally {
     js.context['testExpectsGlobalError'] = false;
   }
   var errors = js.context['testSuppressedGlobalErrors'];
   expect(errors['length'], 1);
   // Clear out the errors;
   js.context['testSuppressedGlobalErrors']['length'] = 0;
 }