Revise duplicate interface check

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
 {
 }
 
 class ImplementsC implements C
 , C  /// 02: compile-time error
 {}
 
+// Spec says: It is a compile-time error if the superclass
+// of a class C appears in the implements clause of C.
 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);
 }