WIP: Close #91 (UselessCode FP)

[KengoTODA]

This PR contains two changes:

• reproduced problem by JUnit
• solve problem by changing ASSUME_ASSERTIONS_ENABLED to false

The 2nd change would affect other detectors, I need to check carefully.

[coveralls]

Coverage decreased (-0.03%) to 56.697% when pulling b5b03c1 on KengoTODA:issue-91 into b3f3117 on spotbugs:master.

[KengoTODA]

As 362c9fb (not in this branch) I created a sub-class of CFG to let specific detector ignores assert, but it failed.
It seems that the problem exists in product architecture: analysis cache doesn't suppose the possibility to have multiple kinds of CFG. I think it's really hard to introduce this way which can keep backward compatibility of other detectors.

So we should choose other methodology, for instance:

• Use SystemProperties.ASSERTIONS_ENABLED instead of ASSUME_ASSERTIONS_ENABLED
• Create an option (system properties?) to switch the value of ASSUME_ASSERTIONS_ENABLED

Note that both of above affects all detectors which depends on CFG.
Any other ideas are welcome :)