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

tests/language/src/Library1NegativeTest.dart

Index: tests/language/src/Library1NegativeTest.dart
===================================================================
--- tests/language/src/Library1NegativeTest.dart	(revision 0)
+++ tests/language/src/Library1NegativeTest.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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.
+
+// This test should fail to load because we are importing two libraries
+// which define the same top level name.
+
+#library("Library1NegativeTest.dart");
+#import("library1.dart");  // Defines top level variable 'foo'
+#import("library2.dart");  // Defines top level variable 'foo'
+
+
+main() {
+  Expect.equals(0, foo1);
+}