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

sdk/lib/_internal/compiler/implementation/util/setlet.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.
 
 library dart2js.util.setlet;
 
 import 'dart:collection' show IterableBase;
 
 class Setlet<E> extends IterableBase<E> implements Set<E> {
   static const _MARKER = const _SetletMarker();
@@ skipping 231 matching lines @@
     _extra = null;
   }
 
   Set<E> union(Set<E> other) => new Set<E>.from(this)..addAll(other);
 
   Setlet<E> intersection(Set<E> other) =>
       new Setlet<E>.from(this.where((e) => other.contains(e)));
 
   Setlet<E> difference(Set<E> other) =>
       new Setlet<E>.from(this.where((e) => !other.contains(e)));
+
+  Setlet<E> toSet() {
+    Setlet<E> result = new Setlet<E>();
+    if (_extra == null) {
+      result._contents = _contents;
+    } else if (_extra == MARKER) {
+      result._extra = MARKER;
+      result._contents = _contents.toSet();
+    } else {
+      result._extra = _extra;
+      result._contents = _contents.toList();
+    }
+    return result;
+  }
 }
 
 class _SetletMarker {
   const _SetletMarker();
   toString() => "-";
 }
 
 class _SetletSingleIterator<E> implements Iterator<E> {
   var _element;
   E _current;
@@ skipping 27 matching lines @@
       if (Setlet._MARKER != candidate) {
         _current = candidate;
         _remaining--;
         return true;
       }
     }
     _current = null;
     return false;
   }
 }