Update a couple more strong mode tests

tests/lib_strong/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.
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
 
 import 'package:expect/minitest.dart';
 
@@ skipping 26 matching lines @@
         upgrader.upgrade(e);
       };
 
       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 as FooElement).initializedField, 666);
     expect(element.text, 'constructed');
 
     js.context.callMethod('validateIsFoo', [element]);
 
-    expect(element.doSomething(), 'didSomething');
-    expect(element.fooCreated, true);
+    expect((element as FooElement).doSomething(), 'didSomething');
+    expect((element as FooElement).fooCreated, true);
   });
 
   test('dart constructor works', () {
     var element = new FooElement();
     expect(element is FooElement, isTrue);
     expect(element.text, 'constructed');
 
     js.context.callMethod('validateIsFoo', [element]);
 
     expect(element.doSomething(), 'didSomething');
   });
 
   test('cannot create upgrader for interfaces', () {
     expect(() {
       document.createElementUpgrader(HtmlElementInterface);
     }, throws);
   });
 
   test('cannot upgrade interfaces', () {
     expect(() {
-      upgrader.upgrade(new HtmlElementInterface());
+      upgrader.upgrade(new HtmlElementInterface()); /// compile-time error
     }, throws);
   });
 
   test('cannot upgrade more than once', () {
     var fooElement = new FooElement();
     expect(() {
       upgrader.upgrade(fooElement);
     }, throws);
   });
 
@@ skipping 23 matching lines @@
     }, throws);
   });
 
   test('cannot create upgrader for built-in types', () {
     expect(() {
       document.createElementUpgrader(HtmlElement);
     }, throws);
   });
 }
 
-class HtmlElementInterface implements HtmlElement {
+class HtmlElementInterface implements HtmlElement { /// compile-time error
   HtmlElementInterface.created();
 }
 
 class CustomDiv extends DivElement {
   CustomDiv.created() : super.created();
 }
 
 class CustomElement extends HtmlElement {
   factory CustomElement() => document.createElement('custom-element');
   CustomElement.created() : super.created();
 }