[dart:html] rename register to registerElement per spec

tests/html/custom/constructor_calls_created_synchronously_test.dart

Index: tests/html/custom/constructor_calls_created_synchronously_test.dart
diff --git a/tests/html/custom/constructor_calls_created_synchronously_test.dart b/tests/html/custom/constructor_calls_created_synchronously_test.dart
index 0eb1f5c8c97772b08f9e5e755eaaa76af756d0ed..256b9f8bdc10cf2ba164c4d93d9001830d81a2c9 100644
--- a/tests/html/custom/constructor_calls_created_synchronously_test.dart
+++ b/tests/html/custom/constructor_calls_created_synchronously_test.dart
@@ -30,7 +30,7 @@ main() {
     return customElementsReady.then((_) {
       if (!registered) {
         registered = true;
-        document.register(A.tag, A);
+        document.registerElement(A.tag, A);
       }
     });
   });
@@ -51,7 +51,8 @@ main() {
   });
 
   test("can extend elements that don't have special prototypes", () {
-    document.register('fancy-section', FancySection, extendsTag: 'section');
+    document.registerElement('fancy-section', FancySection,
+        extendsTag: 'section');
     var fancy = document.createElement('section', 'fancy-section');
     expect(fancy is FancySection, true, reason: 'fancy-section was registered');
     expect(fancy.wasCreated, true, reason: 'FancySection ctor was called');