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

sdk/lib/collection/splay_tree.dart

Index: sdk/lib/collection/splay_tree.dart
diff --git a/sdk/lib/collection/splay_tree.dart b/sdk/lib/collection/splay_tree.dart
index 8e2f5930eff58f4928cc88bcafc1996a6ecf6151..e927e53d2a011ad88f8752d362e1762eecf68f6f 100644
--- a/sdk/lib/collection/splay_tree.dart
+++ b/sdk/lib/collection/splay_tree.dart
@@ -785,7 +785,7 @@ class SplayTreeSet<E> extends _SplayTree<E> with IterableMixin<E>
   }
 
   Set<E> intersection(Set<E> other) {
-    Set<E> result = new SplayTreeSet<E>(_compare, _validKey);
+    Set<E> result = new SplayTreeSet<E>(_comparator, _validKey);
     for (E element in this) {
       if (other.contains(element)) result.add(element);
     }
@@ -793,7 +793,7 @@ class SplayTreeSet<E> extends _SplayTree<E> with IterableMixin<E>
   }
 
   Set<E> difference(Set<E> other) {
-    Set<E> result = new SplayTreeSet<E>(_compare, _validKey);
+    Set<E> result = new SplayTreeSet<E>(_comparator, _validKey);
     for (E element in this) {
       if (!other.contains(element)) result.add(element);
     }
@@ -804,8 +804,12 @@ class SplayTreeSet<E> extends _SplayTree<E> with IterableMixin<E>
     return _clone()..addAll(other);
   }
 
+  SplayTreeSet<E> cloneEmpty() {
+    return new SplayTreeSet<E>(_comparator, _validKey);
+  }
+
   SplayTreeSet<E> _clone() {
-    var set = new SplayTreeSet<E>(_compare, _validKey);
+    var set = new SplayTreeSet<E>(_comparator, _validKey);
     set._count = _count;
     set._root = _cloneNode(_root);
     return set;
@@ -825,4 +829,6 @@ class SplayTreeSet<E> extends _SplayTree<E> with IterableMixin<E>
   }
 
   void clear() { _clear(); }
+
+  Set<E> toSet() => _clone();
 }