Fix CssClassSet methods to accept Set, not Collection.

tools/dom/src/CssClassSet.dart

Index: tools/dom/src/CssClassSet.dart
diff --git a/tools/dom/src/CssClassSet.dart b/tools/dom/src/CssClassSet.dart
index 8d04493d596e0edb233c886290893911d2f27e45..dbc8afc30a5bfcc87a7bb42a0a2ce09e586740f9 100644
--- a/tools/dom/src/CssClassSet.dart
+++ b/tools/dom/src/CssClassSet.dart
@@ -108,13 +108,13 @@ 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;