tests/html: removed usage of deprecated unittest features

tests/html/url_test.dart

Index: tests/html/url_test.dart
diff --git a/tests/html/url_test.dart b/tests/html/url_test.dart
index 32143990fa481f43483d25b27bf09c8858430045..6852fc68d5eb1ee565eecfb3ed53ac21a482c69f 100644
--- a/tests/html/url_test.dart
+++ b/tests/html/url_test.dart
@@ -46,9 +46,7 @@ main() {
         expect(img.complete, true);
       }));
       img.onError.listen((_) {
-        guardAsync(() {
-          expect(true, isFalse, reason: 'URL failed to load.');
-        });
+        fail('URL failed to load.');
       });
       img.src = url;
     });
@@ -64,9 +62,7 @@ main() {
       img.onError.listen(expectAsync((_) {
       }));
       img.onLoad.listen((_) {
-        guardAsync(() {
-          expect(true, isFalse, reason: 'URL should not have loaded.');
-        });
+        fail('URL should not have loaded.');
       });
       img.src = url;
     });