Add test where X is defined differentlty in co-importing libraries

tests/language/src/LibrarySameNameUsedLib2.dart

Index: tests/language/src/LibrarySameNameUsedLib2.dart
diff --git a/tests/language/src/LibrarySameNameUsedLib2.dart b/tests/language/src/LibrarySameNameUsedLib2.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d50e569352545b8ff725c0e5627c4e2fe3079640
--- /dev/null
+++ b/tests/language/src/LibrarySameNameUsedLib2.dart
@@ -0,0 +1,11 @@
+// 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('lib2');
+#import('LibrarySameNameUsedLib1.dart', prefix:'lib1');  // for interface X.
+
+class X implements lib1.X {
+  X();
+  toString() => 'lib2.X';
+}