Revise duplicate interface check

tests/language/duplicate_interface_negative_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.
+// Check fail because of cycles in super class relationship.
+
+import "duplicate_interface_lib.dart" as alib;
+import "duplicate_interface_lib.dart" show InterfA;
+
+
+// Expect error since InterfA and alib.InterfA refer to the same interface.
+class Foo implements InterfA, alib.InterfA { } /// compile-time error
+
+
+main() {
+  Expect.isTrue(new Foo() is InterfA);
+  Expect.isTrue(new Foo() is alib.InterfA);
+}
+