Revert "Report errors on duplicate export."

tests/language/duplicate_import_libc.dart

Index: tests/language/duplicate_import_libc.dart
diff --git a/tests/language/duplicate_import_libc.dart b/tests/language/duplicate_import_libc.dart
deleted file mode 100644
index d1f1b41cfe13f16b4c05709922deade56dc3c9ba..0000000000000000000000000000000000000000
--- a/tests/language/duplicate_import_libc.dart
+++ /dev/null
@@ -1,11 +0,0 @@
-// 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.
-
-library libc;
-
-var field;
-
-void method() {}
-
-class Class {}