Report errors on repeated interfaces.

tests/language/class_cycle_test.dart

 // Copyright (c) 2012, 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.
 // Check fail because of cycles in super class relationship.
 
 class Foo implements Bar {
 }
 
 class C {
 }
 
 class Bar
     extends Foo  /// 00: compile-time error
     implements Foo  /// 01: compile-time error
-    implements C, C  /// 02: compile-time error
-    extends C implements C  /// 03: compile-time error
 {
 }
 
+class ImplementsC implements C
+, C  /// 02: compile-time error
+{}
+
+class ExtendsC extends C
+implements C  /// 03: compile-time error
+{}
+
 main() {
   Expect.isTrue(new Foo() is Foo);
+  Expect.isTrue(new ImplementsC() is C);
+  Expect.isTrue(new ExtendsC() is C);
 }