Enforce token syntax for CssClassSet arguments

tools/dom/src/dart2js_CssClassSet.dart

Index: tools/dom/src/dart2js_CssClassSet.dart
diff --git a/tools/dom/src/dart2js_CssClassSet.dart b/tools/dom/src/dart2js_CssClassSet.dart
index 783828b6a3876fee38b8d3a4a06382d8491d9c67..cf5fa33660748dc256431f16a4a2704dcf39ccc8 100644
--- a/tools/dom/src/dart2js_CssClassSet.dart
+++ b/tools/dom/src/dart2js_CssClassSet.dart
@@ -100,7 +100,7 @@ class _ElementCssClassSet extends CssClassSetImpl {
     _element.className = '';
   }
 
-  bool contains(String value) {
+  bool contains(Object value) {
     return _contains(_element, value);
   }
 
@@ -136,8 +136,8 @@ class _ElementCssClassSet extends CssClassSetImpl {
     _removeWhere(_element, test, false);
   }
 
-  static bool _contains(Element _element, String value) {
-    return _classListContains(_classListOf(_element), value);
+  static bool _contains(Element _element, Object value) {
+    return value is String && _classListContains(_classListOf(_element), value);
   }
 
   static bool _add(Element _element, String value) {