Add negative test for detecting error when the same library is imported twice.

tests/language/src/library2.dart

Index: tests/language/src/library2.dart
===================================================================
--- tests/language/src/library2.dart	(revision 0)
+++ tests/language/src/library2.dart	(revision 0)
@@ -0,0 +1,9 @@
+// Copyright (c) 2011, 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("library2.dart");
+
+var foo;
+var foo1 = 0;