Enable these checks in custom tests

tests/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.
 
 library document_register_basic_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 
 class Foo extends HtmlElement {
@@ skipping 74 matching lines @@
     document.body.append(container);
     container.innerHtml = "<x-foo></x-foo>";
     var parsedFoo = container.firstChild;
 
     expect(parsedFoo is Foo, isTrue);
     expect(parsedFoo.tagName, "X-FOO");
 
     // Ensuring the wrapper is retained
     var someProperty = new Expando();
     someProperty[parsedFoo] = "hello";
-    // TODO(vsm): Fix this in Dartium.
-    // expect(someProperty[container.firstChild], someProperty[parsedFoo]);
-    // expect(container.firstChild, parsedFoo);
+    expect(container.firstChild, parsedFoo);
+    expect(someProperty[container.firstChild], someProperty[parsedFoo]);
 
     // Having another constructor
     document.register(Bar.tag, Bar);
     var createdBar = new Bar();
     expect(createdBar is Bar, isTrue);
     expect(createdBar is Foo, isFalse);
     expect(createdBar.tagName, "X-BAR");
 
     // Having a subclass
     document.register(Baz.tag, Baz);
@@ skipping 15 matching lines @@
     expect(container.firstChild.tagName, "X-BAR");
     expect(container.lastChild is Bar, isTrue);
     expect(container.lastChild.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");
   });
 }