Use a trail instead of a mark bit when processing recursive types in the VM

tests/language/cyclic_type2_test.dart

Index: tests/language/cyclic_type2_test.dart
===================================================================
--- tests/language/cyclic_type2_test.dart	(revision 0)
+++ tests/language/cyclic_type2_test.dart	(revision 0)
@@ -0,0 +1,31 @@
+// 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.
+
+// Tests self referencing types.
+
+import "package:expect/expect.dart";
+
+class Base<U, V> {
+  get u => U;
+  get v => V;
+}
+
+class Derived1<U, V> extends Base<Derived1<U, V>,
+                                  Derived1<Derived2<V, U>, Derived2>> {}
+
+class Derived2<U, V> extends Base<Derived2<U, V>,
+                                  Derived2<Derived1<V, U>, Derived1>> {}
+
+main() {
+  var d = new Derived1<Derived1, Derived2>();
+  Expect.equals("Derived1<Derived1, Derived2>", d.u.toString());
+  Expect.equals("Derived1<Derived2<Derived2, Derived1>, Derived2>",
+                d.v.toString());
+  Expect.isTrue(d is Derived1<Derived1, Derived2>);
+  Expect.isFalse(d is Derived1<Derived1, Derived1>);
+  Expect.isTrue(d is Base<Derived1<Derived1, Derived2>,
+                          Derived1<Derived2<Derived2, Derived1>, Derived2>>);
+  Expect.isFalse(d is Base<Derived1<Derived1, Derived2>,
+                           Derived1<Derived2<Derived2, Derived2>, Derived2>>);
+}