[dart:html] rename register to registerElement per spec

tests/html/custom/element_upgrade_test.dart

 // Copyright (c) 2014, 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 register_element_proxy_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
 import 'package:unittest/html_config.dart';
@@ skipping 22 matching lines @@
   var registered = false;
   var upgrader;
   setUp(() => customElementsReady.then((_) {
     if (!registered) {
       registered = true;
       upgrader = document.createElementUpgrader(FooElement);
       js.context['upgradeListener'] = (e) {
         upgrader.upgrade(e);
       };
 
-      document.register('custom-element', CustomElement);
+      document.registerElement('custom-element', CustomElement);
     }
   }));
 
   test('created gets proxied', () {
     var element = document.createElement(FooElement.tag);
     expect(element is FooElement, isTrue);
     expect(element.initializedField, 666);
     expect(element.text, 'constructed');
 
     js.context.callMethod('validateIsFoo', [element]);
@@ skipping 69 matching lines @@
 }
 
 class CustomDiv extends DivElement {
   CustomDiv.created() : super.created();
 }
 
 class CustomElement extends HtmlElement {
   factory CustomElement() => document.createElement('custom-element');
   CustomElement.created() : super.created();
 }