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

tests/html/documentfragment_test.dart

Index: tests/html/documentfragment_test.dart
diff --git a/tests/html/documentfragment_test.dart b/tests/html/documentfragment_test.dart
index f400804460c9ff3e070aa469d0e0620617bdcf73..1699897ee6a8bf193bc9d5989e4b8ef19f8ab10a 100644
--- a/tests/html/documentfragment_test.dart
+++ b/tests/html/documentfragment_test.dart
@@ -81,6 +81,7 @@ main() {
     var emptyFragment = new DocumentFragment();
     expectUnsupported(() => emptyFragment.attributes = {});
     expectUnsupported(() => emptyFragment.classes = []);
+    expectUnsupported(() => emptyFragment.classes.add('foo'));
     expectUnsupported(() => emptyFragment.dataAttributes = {});
     expectUnsupported(() => emptyFragment.contentEditable = "true");
     expectUnsupported(() => emptyFragment.dir);