RFR: add javac and javap support for the ACC_NULL_CHECKED flag [v2]

[Vicente Romero]

> adding support for the ACC_NULL_CHECKED flag

Vicente Romero has updated the pull request incrementally with two additional commits since the last revision:

 - tests
 - minor fix

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/2017/files
  - new: https://git.openjdk.org/valhalla/pull/2017/files/56a385ac..77364f33

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=2017&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=2017&range=00-01

  Stats: 29 lines in 3 files changed: 5 ins; 21 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/2017.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2017/head:pull/2017

PR: https://git.openjdk.org/valhalla/pull/2017