Report errors on duplicate export.

tests/language/duplicate_export_negative_test.dart

Index: tests/language/duplicate_export_negative_test.dart
diff --git a/tests/language/duplicate_export_negative_test.dart b/tests/language/duplicate_export_negative_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..51a2294d7f9bdbc137ca8eb0d76209577db1aaa5
--- /dev/null
+++ b/tests/language/duplicate_export_negative_test.dart
@@ -0,0 +1,8 @@
+// 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.
+
+import 'duplicate_import_libd.dart';
+
+void main() {
+}