Fix CssClassSet methods to accept Set, not Collection.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 9eefda416783f43010109d53376ad19977607fe0..b1165f68342d888f1f8f06e1ff264af15a00341a 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -32498,13 +32498,13 @@ 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;