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

tests/language/src/LibrarySameNameUsedTest.dart

+// 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.
+
+// 'X' is defined as an interface in lib1 and a class implementing that
+// interface in lib2.  Use of #import prefixes should allow this.
+
+#library('main')
+#import("LibrarySameNameUsedLib1.dart");
+
+main() {
+  var x = makeX();
+  Expect.equals('lib2.X', '$x');
+}