Fixed bugs identified by Jacks: Round 1.  Added relevant test cases.

- null + null should be illegal.
- int i = 1L; should be illegal.
- Duplicate superinterface should be illegal.
- Interface constant flags must be public static final and nothing else.
- super() constructor call to local class declared in static context
does not have a qualifier.
- Duplicate labels should be illegal.
- LocalClassDecl should be Stmt, not CompoundStmt, to obtain correct
reachability error.
- Case value should be assignable to switch type.
- synchronize (null) {} is illegal.
- Local class is an inner class, regardless whether it is declared
within a static context.
- Method cannot be both native and strictfp.
- Field cannot be both final and volatile.
- Class cannot be both abstract and final.