Some browsers don't support TrackElement

dart/tests/html/track_element_constructor_test.dart

Index: dart/tests/html/track_element_constructor_test.dart
diff --git a/dart/tests/html/track_element_constructor_test.dart b/dart/tests/html/track_element_constructor_test.dart
index a7a12f3ec25a62ee4f348e4008b76f5a3f24b89d..883dabdbf862565c14a47785fcae23b243761d2f 100644
--- a/dart/tests/html/track_element_constructor_test.dart
+++ b/dart/tests/html/track_element_constructor_test.dart
@@ -17,6 +17,7 @@ import 'package:unittest/html_config.dart';
 void main() {
   useHtmlConfiguration();
   test('', () {
+    if (!TrackElement.supported) return;
     document.body.append(new TrackElement()..defaultValue = true);
     if (!document.query('track').defaultValue) {
       throw 'Expected default value to be true';