Fix bug in SplayTreeSet when removeWhere removes all elements.

tests/corelib/splay_tree_test.dart

 // Copyright (c) 2011, 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.
 
 // Dart test for Splaytrees.
 library splay_tree_test;
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 
@@ skipping 33 matching lines @@
   Expect.equals(4, tree.lastKeyBefore(5));
   Expect.equals(7, tree.firstKeyAfter(5));
 
   Expect.equals(5, tree.lastKeyBefore(7));
   Expect.equals(null, tree.firstKeyAfter(7));
 
   Expect.equals(5, tree.lastKeyBefore(6));
   Expect.equals(7, tree.firstKeyAfter(6));
 
   regressRemoveWhere();
+  regressRemoveWhere2();
 }
 
 void regressRemoveWhere() {
   // Regression test. Fix in https://codereview.chromium.org/148523006/
   var t = new SplayTreeSet();
   t.addAll([1,3,5,7,2,4,6,8,0]);
   var seen = new List<bool>.filled(9, false);
   t.removeWhere((x) {
     // Called only once per element.
     Expect.isFalse(seen[x], "seen $x");
     seen[x] = true;
     return x.isOdd;
   });
 }
+
+void regressRemoveWhere2() {
+  // Regression test for http://dartbug.com/18676
+  // Removing all elements with removeWhere causes error.
+
+  var t = new SplayTreeSet();
+  t.addAll([1,2,3,4,5]);
+  t.removeWhere((_) => true);  // Should not throw.
+  Expect.isTrue(t.isEmpty);
+  t.addAll([1,2,3,4,5]);
+  t.retainWhere((_) => false);  // Should not throw.
+  Expect.isTrue(t.isEmpty);
+}