Update a couple more strong mode tests

tests/lib_strong/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.
 
 import 'dart:html';
 
 import 'package:expect/minitest.dart';
 
 import '../utils.dart';
 
@@ skipping 21 matching lines @@
   get thisIsAlsoACustomClass => true;
 }
 
 class BadB {
 }
 
 abstract class BadC extends HtmlElement {
   BadC.created() : super.created();
 }
 
-class BadF implements HtmlElement {
+class BadF implements HtmlElement { /// compile-time error
   static final tag = 'x-tag-f';
   factory BadF() => new Element.tag(tag);
 }
 
 main() {
   // Adapted from Blink's fast/dom/custom/document-register-basic test.
 
   setUp(() => customElementsReady);
 
   test('Testing document.registerElement() basic behaviors', () {
@@ skipping 40 matching lines @@
 
     // Parser initiated instantiation
     var container = new DivElement()..id = "container";
     document.body.append(container);
     container.setInnerHtml("<x-foo></x-foo>",
         treeSanitizer: new NullTreeSanitizer());
     upgradeCustomElements(container);
     var parsedFoo = container.firstChild;
 
     expect(parsedFoo is Foo, isTrue);
-    expect(parsedFoo.tagName, "X-FOO");
+    expect((parsedFoo as Foo).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]);
 
     // Having another constructor
     document.registerElement(Bar.tag, Bar);
     var createdBar = new Bar();
@@ skipping 13 matching lines @@
     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());
     upgradeCustomElements(container);
     expect(container.firstChild is Bar, isTrue);
-    expect(container.firstChild.tagName, "X-BAR");
+    expect((container.firstChild as Bar).tagName, "X-BAR");
     expect(container.lastChild is Bar, isTrue);
-    expect(container.lastChild.tagName, "X-BAR");
+    expect((container.lastChild as Bar).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");
   });
 }