Change Set.toSet to always return a set with the same behavior.

sdk/lib/html/html_common/css_class_set.dart

Index: sdk/lib/html/html_common/css_class_set.dart
diff --git a/sdk/lib/html/html_common/css_class_set.dart b/sdk/lib/html/html_common/css_class_set.dart
index a81497e401f08207a316f332de3962bba0dc4049..ef6f9680073cf24a2893cdb224287f00d02cf12a 100644
--- a/sdk/lib/html/html_common/css_class_set.dart
+++ b/sdk/lib/html/html_common/css_class_set.dart
@@ -172,6 +172,10 @@ abstract class CssClassSetImpl implements CssClassSet {
   Set<String> difference(Set<String> other) =>
     readClasses().difference(other);
 
+  // Do overwrite this method in subclasses if it is not the type of set
+  // returned by readClasses.
+  Set<String> cloneEmpty() => new LinkedHashSet<String>();
+
   String get first => readClasses().first;
   String get last => readClasses().last;
   String get single => readClasses().single;