Fix bug in SplayTreeSet when removeWhere removes all elements.

tests/corelib/splay_tree_test.dart

Index: tests/corelib/splay_tree_test.dart
diff --git a/tests/corelib/splay_tree_test.dart b/tests/corelib/splay_tree_test.dart
index ee3083df577fc02a6a90d0514e0950e959e56cc9..2557b54012197fe6c6cf0693f20a04e9f78a79ec 100644
--- a/tests/corelib/splay_tree_test.dart
+++ b/tests/corelib/splay_tree_test.dart
@@ -51,6 +51,7 @@ main() {
   Expect.equals(7, tree.firstKeyAfter(6));
 
   regressRemoveWhere();
+  regressRemoveWhere2();
 }
 
 void regressRemoveWhere() {
@@ -65,3 +66,16 @@ void regressRemoveWhere() {
     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);
+}