[dart:html] rename register to registerElement per spec

tests/html/custom/regress_194523002_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.
 
 // Regression test for CL 194523002.
 
 library js_custom_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
@@ skipping 10 matching lines @@
   useHtmlConfiguration();
 
   // Adapted from Blink's
   // fast/dom/custom/constructor-calls-created-synchronously test.
 
   var registered = false;
   setUp(() {
     return customElementsReady.then((_) {
       if (!registered) {
         registered = true;
-        document.register(A.tag, A);
+        document.registerElement(A.tag, A);
       }
     });
   });
 }