Detect if webkitNotifications is available and remove webkit prefix (replaced

tests/html/notifications_test.dart

+library NotificationsTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_individual_config.dart';
+import 'dart:html';
+
+main() {
+  useHtmlIndividualConfiguration();
+
+  group('supported', () {
+    test('supported', () {
+      expect(NotificationCenter.supported, true);
+    });
+  });
+
+  group('unsupported throws', () {
+    test('createNotification', () {
+      var expectation = NotificationCenter.supported ? returnsNormally : throws;
+      expect(() { window.notifications.createNotification; }, expectation);
+    });
+  });
+
+  group('webkitNotifications', () {
+    if (NotificationCenter.supported) {
+      test('DomException', () {
+        try {
+          window.notifications.createNotification('', '', '');
+        } on DomException catch (e) {
+          expect(e.name, DomException.SECURITY);
+        }
+      });
+    }
+  });
+}
+