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

pkg/collection/lib/wrappers.dart

Index: pkg/collection/lib/wrappers.dart
diff --git a/pkg/collection/lib/wrappers.dart b/pkg/collection/lib/wrappers.dart
index 0fcc8c23e80a42f6e86c84ebea3988f512cfca02..a81572befcd8d8168ceea2f2e228fe0bd75f5566 100644
--- a/pkg/collection/lib/wrappers.dart
+++ b/pkg/collection/lib/wrappers.dart
@@ -242,6 +242,10 @@ class DelegatingSet<E> extends DelegatingIterable<E> implements Set<E> {
   }
 
   Set<E> union(Set<E> other) => _setBase.union(other);
+
+  Set<E> toSet() => new DelegatingSet<E>(_setBase.toSet());
+
+  Set<E> cloneEmpty() => new DelegatingSet<E>(_setBase.cloneEmpty());
 }
 
 /**