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

sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.helpers;
 
 /**
  * The expensive set is a data structure useful for tracking down
  * excessive memory usage due to large sets. It acts as an ordinary
  * hash set, but it uses 10 times more memory (by default).
@@ skipping 102 matching lines @@
   void retainAll(Iterable objectsToRetain) {
     Set retainSet;
     if (objectsToRetain is Set) {
       retainSet = objectsToRetain;
     } else {
       retainSet = objectsToRetain.toSet();
     }
     retainWhere(retainSet.contains);
   }
 
+  Set<E> toSet() {
+    var result = new ExpensiveSet<E>(_sets.length);
+    for (int i = 0; i < _sets.length; i++) {
+      result._sets[i] = _sets[i].toSet();
+    }
+    return result;
+  }
+
+  Set<E> cloneEmpty() {
+    return new ExpensiveSet<E>(_sets.length)
+  }
+
   String toString() => "expensive(${_sets[0]}x${_sets.length})";
 }