Report errors on duplicate export.

tests/language/duplicate_import_libc.dart

Index: tests/language/duplicate_import_libc.dart
diff --git a/tests/language/export_helper2.dart b/tests/language/duplicate_import_libc.dart
similarity index 79%
copy from tests/language/export_helper2.dart
copy to tests/language/duplicate_import_libc.dart
index 69a7faba9c100c9e71592634fbdcfbd9e2eec66a..d1f1b41cfe13f16b4c05709922deade56dc3c9ba 100644
--- a/tests/language/export_helper2.dart
+++ b/tests/language/duplicate_import_libc.dart
@@ -2,8 +2,10 @@
 // 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;
+library libc;
 
-class ReExported {
+var field;
 
-}
+void method() {}
+
+class Class {}