fix custom elements that extend tags like "section"

tests/html/custom/constructor_calls_created_synchronously_test.dart

Index: tests/html/custom/constructor_calls_created_synchronously_test.dart
diff --git a/tests/html/custom/constructor_calls_created_synchronously_test.dart b/tests/html/custom/constructor_calls_created_synchronously_test.dart
index f9f242d8a7dd7b43f671ba7c646391c4aaabfc9f..d2bc3afdf746ff66e79a7aaca02216c4ad573d67 100644
--- a/tests/html/custom/constructor_calls_created_synchronously_test.dart
+++ b/tests/html/custom/constructor_calls_created_synchronously_test.dart
@@ -49,4 +49,18 @@ main() {
     expect(A.ncallbacks, 2);
     expect(b is A, isTrue);
   });
+
+  test("can extend elements that don't have special prototypes", () {
+    document.register('fancy-section', FancySection, 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');
+  });
+}
+
+class FancySection extends HtmlElement {
+  bool wasCreated = false;
+  FancySection.created() : super.created() {
+    wasCreated = true;
+  }
 }