Report errors on duplicate export.

tests/language/duplicate_import_libb.dart

Index: tests/language/duplicate_import_libb.dart
diff --git a/sdk/lib/coreimpl/coreimpl.dart b/tests/language/duplicate_import_libb.dart
similarity index 73%
copy from sdk/lib/coreimpl/coreimpl.dart
copy to tests/language/duplicate_import_libb.dart
index 067ea2fe3dd668ef47b4c870d1569db4c8a0cbf7..9b87595f24141781591ae3694821cc882bb1a711 100644
--- a/sdk/lib/coreimpl/coreimpl.dart
+++ b/tests/language/duplicate_import_libb.dart
@@ -2,8 +2,12 @@
 // 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("dart:coreimpl");
+library libb;
 
-#import("dart:collection");
+var field;
 
-#source("regexp.dart");
+void method() {}
+
+class Class {}
+
+class methodOrClass {}