Revise duplicate interface check

tests/language/duplicate_interface_test.dart

Index: tests/language/duplicate_interface_test.dart
===================================================================
--- tests/language/duplicate_interface_test.dart	(revision 0)
+++ tests/language/duplicate_interface_test.dart	(revision 0)
@@ -0,0 +1,18 @@
+// 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.
+// Check fail because of cycles in super class relationship.
+
+import "duplicate_interface_lib.dart" as alib;
+
+class InterfB { }
+
+// Ok since InterfB and alib.InterfB are not the same interface
+class Foo implements InterfB , alib.InterfB { }
+
+
+main() {
+  Expect.isTrue(new Foo() is InterfB);
+  Expect.isTrue(new Foo() is alib.InterfB);
+}
+