If you're interesting in doing litigation, go ahead and specialize. Do not, however, neglect taking the core classes that firms will expect you to have.
Litigation is a popular place to start after graduation. One reason for this is that litigation involves the type of skills you develop in law school (legal research and writing).
The only harm in specializing is that you may seem unconvincing should you decide to do something else. All things being equal, a real estate department will probably hire the student who took real estate classes over the one who specialized in litigation, regardless of how much that student says she wants to practice real estate.