Fix CssClassSet methods to accept Set, not Collection.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index fa27d0d2b71cbe54b265eb70988b6bcab64da585..87c5c463e50e3767e6881444aaa0a1bb9f47454e 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -30182,13 +30182,13 @@ abstract class CssClassSet implements Set<String> {
   bool containsAll(Iterable<String> collection) =>
     readClasses().containsAll(collection);
 
-  Set<String> intersection(Collection<String> other) =>
+  Set<String> intersection(Set<String> other) =>
     readClasses().intersection(other);
 
-  Set<String> union(Collection<String> other) =>
+  Set<String> union(Set<String> other) =>
     readClasses().union(other);
 
-  Set<String> difference(Collection<String> other) =>
+  Set<String> difference(Set<String> other) =>
     readClasses().difference(other);
 
   String get first => readClasses().first;