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

sdk/lib/core/set.dart

Index: sdk/lib/core/set.dart
diff --git a/sdk/lib/core/set.dart b/sdk/lib/core/set.dart
index 271b3898638d7923220fe51e56e2dcaaff8291f8..2440c69f88bf488462a7c7d69c69c090c9bd417a 100644
--- a/sdk/lib/core/set.dart
+++ b/sdk/lib/core/set.dart
@@ -166,4 +166,14 @@ abstract class Set<E> extends IterableBase<E> implements EfficientLength {
    * Removes all elements in the set.
    */
   void clear();
+
+  /* Creates a [Set] with the same elements and behavior as this `Set`.
+   *
+   * The returned set behaves the same as this set
+   * with regard to adding and removing elements.
+   * It initially contains the same elements.
+   * If this set specifies an ordering of the elements,
+   * the returned set will have the same order.
+   */
+  Set<E> toSet();
 }