Disallow cyclic mixins.

tests/language/mixin_illegal_cycles_test.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.
+
+class M { }
+class M0 extends Object with M0 { }  /// 01: compile-time error
+typedef M1 = Object with M1;         /// 02: compile-time error
+
+typedef M2 = Object with M3;         /// 03: compile-time error
+typedef M3 = Object with M2;         /// 03: continued
+
+typedef M4 = Object with M5;         /// 04: compile-time error
+typedef M5 = Object with M6;         /// 04: continued
+typedef M6 = Object with M4;         /// 04: continued
+
+class M7 extends Object with M8 { }  /// 05: compile-time error
+class M8 extends Object with M7 { }  /// 05: continued
+
+main() {
+  new M0();  /// 01: continued
+
+  new M1();  /// 02: continued
+
+  new M2();  /// 03: continued
+  new M3();  /// 03: continued
+
+  new M4();  /// 04: continued
+  new M5();  /// 04: continued
+  new M6();  /// 04: continued
+
+  new M7();  /// 05: continued
+  new M8();  /// 05: continued
+}