Enable these checks in custom tests

tests/html/custom/document_register_basic_test.dart

Index: tests/html/custom/document_register_basic_test.dart
diff --git a/tests/html/custom/document_register_basic_test.dart b/tests/html/custom/document_register_basic_test.dart
index a6aa7462e25d99c4cfb4af36465d77484f1ec65c..718358afbe05b40f14df877a4b1758fb4131c005 100644
--- a/tests/html/custom/document_register_basic_test.dart
+++ b/tests/html/custom/document_register_basic_test.dart
@@ -92,9 +92,8 @@ main() {
     // 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);