Enforce token syntax for CssClassSet arguments

tests/html/svgelement_test.dart

Index: tests/html/svgelement_test.dart
diff --git a/tests/html/svgelement_test.dart b/tests/html/svgelement_test.dart
index 776ecf8745ad7c8b88f376de0f78c8a86d0528c4..2b573ca97cc477ed5afd8970af1c701ed970164e 100644
--- a/tests/html/svgelement_test.dart
+++ b/tests/html/svgelement_test.dart
@@ -424,6 +424,33 @@ main() {
       classes.toggle('foo');
       expect(el.classes.length, 0);
     });
+
+    test('classes-add-bad', () {
+      var el = new svg.CircleElement();
+      expect(() => el.classes.add(''), throws);
+      expect(() => el.classes.add('foo bar'), throws);
+    });
+    test('classes-remove-bad', () {
+      var el = new svg.CircleElement();
+      expect(() => el.classes.remove(''), throws);
+      expect(() => el.classes.remove('foo bar'), throws);
+    });
+    test('classes-toggle-token', () {
+      var el = new svg.CircleElement();
+      expect(() => el.classes.toggle(''), throws);
+      expect(() => el.classes.toggle('', true), throws);
+      expect(() => el.classes.toggle('', false), throws);
+      expect(() => el.classes.toggle('foo bar'), throws);
+      expect(() => el.classes.toggle('foo bar', true), throws);
+      expect(() => el.classes.toggle('foo bar', false), throws);
+    });
+    test('classes-contains-bad', () {
+      var el = new svg.CircleElement();
+      // Non-strings => false, strings must be valid tokens.
+      expect(el.classes.contains(1), isFalse);
+      expect(() => el.classes.contains(''), throws);
+      expect(() => el.classes.contains('foo bar'), throws);
+    });
   });
 
   group('getBoundingClientRect', () {