Added test case for syntax highlighting rule for DS digits

@Benglin PTAL
```static``` stuff stays alive unless explicitly set to ```null```, and typically it is really hard to know when (i.e. when you set a ```static``` member to ```null```, how can you be sure that no other calls to its accessor take place after it is set to ```null``` and turn it back to alive again?) If these ```static``` member refers to something else, those things stay alive in memory and get promoted to avoid garbage collection. This ultimately result in memory leaks. Almost all the leaking issue we see today are due to having a ```static``` objects which unfortunately hold other things alive.