Adding .toggle method to Element.classes to match the JS element.classList.toggle method.

tests/html/element_classes_test.dart

Index: tests/html/element_classes_test.dart
diff --git a/tests/html/element_classes_test.dart b/tests/html/element_classes_test.dart
index 7b325bb6749392029c927078dcaddac3a36f5b90..e4a8d50945647a7ff700d35e9dd5082238f3507a 100644
--- a/tests/html/element_classes_test.dart
+++ b/tests/html/element_classes_test.dart
@@ -123,6 +123,14 @@ main() {
     Expect.setEquals(['foo', 'baz'], classes);
   });
 
+  test('toggle', () {
+    final classes = makeClassSet();
+    classes.toggle('bar');
+    Expect.setEquals(['foo', 'baz'], classes);
+    classes.toggle('qux');
+    Expect.setEquals(['foo', 'baz', 'qux'], classes);
+  });
+
   test('addAll', () {
     final classes = makeClassSet();
     classes.addAll(['bar', 'qux', 'bip']);