Report error on duplicate supertypes

tests/language/src/DuplicateImplementsTest.dart

Index: tests/language/src/DuplicateImplementsTest.dart
===================================================================
--- tests/language/src/DuplicateImplementsTest.dart	(revision 0)
+++ tests/language/src/DuplicateImplementsTest.dart	(revision 0)
@@ -0,0 +1,19 @@
+// 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 that duplicate types in implements/extends list are
+// compile-time errors.
+
+interface I { }
+interface J { }
+interface K<T> { }
+
+class X implements I, J, I { }               /// 01: compile-time error
+class X implements J, I, K<int>, K<int> { }  /// 02: compile-time error
+
+interface Z extends I, J, J { }              /// 03: compile-time error
+interface Z extends K<int>, K<int> { }       /// 04: compile-time error
+
+main() {
+  return null;
+}