Report errors on duplicate export.

tests/language/duplicate_import_libb.dart

Index: tests/language/duplicate_import_libb.dart
diff --git a/tests/language/duplicate_import_libb.dart b/tests/language/duplicate_import_libb.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9b87595f24141781591ae3694821cc882bb1a711
--- /dev/null
+++ b/tests/language/duplicate_import_libb.dart
@@ -0,0 +1,13 @@
+// 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 libb;
+
+var field;
+
+void method() {}
+
+class Class {}
+
+class methodOrClass {}