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/export_helper2.dart b/tests/language/duplicate_export_negative_test.dart
similarity index 80%
copy from tests/language/export_helper2.dart
copy to tests/language/duplicate_export_negative_test.dart
index 69a7faba9c100c9e71592634fbdcfbd9e2eec66a..51a2294d7f9bdbc137ca8eb0d76209577db1aaa5 100644
--- a/tests/language/export_helper2.dart
+++ b/tests/language/duplicate_export_negative_test.dart
@@ -2,8 +2,7 @@
 // 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.
 
-library export_helper2;
-
-class ReExported {
+import 'duplicate_import_libd.dart';
 
+void main() {
 }