Reapply remove upgradeCustomElement methods.

tests/html/custom/document_register_basic_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 document_register_basic_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import '../utils.dart';
 
@@ skipping 73 matching lines @@
     // Native method
     var childDiv = new DivElement();
     createdFoo.append(childDiv);
     expect(createdFoo.lastChild, childDiv);
 
     // Parser initiated instantiation
     var container = new DivElement()..id = "container";
     document.body.append(container);
     container.setInnerHtml("<x-foo></x-foo>",
         treeSanitizer: new NullTreeSanitizer());
-    Platform.upgradeCustomElements(container);
+    upgradeCustomElements(container);
     var parsedFoo = container.firstChild;
 
     expect(parsedFoo is Foo, isTrue);
     expect(parsedFoo.tagName, "X-FOO");
 
     // Ensuring the wrapper is retained
     var someProperty = new Expando();
     someProperty[parsedFoo] = "hello";
     expect(container.firstChild, parsedFoo);
     expect(someProperty[container.firstChild], someProperty[parsedFoo]);
@@ skipping 15 matching lines @@
     // With irregular cases
     var createdUpperBar = new Element.tag("X-BAR");
     var createdMixedBar = new Element.tag("X-Bar");
     expect(createdUpperBar is Bar, isTrue);
     expect(createdUpperBar.tagName, "X-BAR");
     expect(createdMixedBar is Bar, isTrue);
     expect(createdMixedBar.tagName, "X-BAR");
 
     container.setInnerHtml("<X-BAR></X-BAR><X-Bar></X-Bar>",
         treeSanitizer: new NullTreeSanitizer());
-    Platform.upgradeCustomElements(container);
+    upgradeCustomElements(container);
     expect(container.firstChild is Bar, isTrue);
     expect(container.firstChild.tagName, "X-BAR");
     expect(container.lastChild is Bar, isTrue);
     expect(container.lastChild.tagName, "X-BAR");
 
     // Constructors shouldn't interfere with each other
     expect((new Foo()).tagName, "X-FOO");
     expect((new Bar()).tagName, "X-BAR");
     expect((new Baz()).tagName, "X-BAZ");
   });
 }