Update a couple more strong mode tests

tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart

Index: tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart
diff --git a/tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart b/tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart
index 256b9f8bdc10cf2ba164c4d93d9001830d81a2c9..b621efc9d86c13768fbcd23343df5fa5db276f32 100644
--- a/tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart
+++ b/tests/lib_strong/html/custom/constructor_calls_created_synchronously_test.dart
@@ -55,7 +55,7 @@ main() {
         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');
+    expect((fancy as FancySection).wasCreated, true, reason: 'FancySection ctor was called');
   });
 }