RFR: 8224922: Access JavaFileObject from Element(s) [v8]

[Joe Darcy]

> Initial review to get some comments on the shape of the API. Tests are needed of course, as well as some tuning of the spec to better describe differences in behavior when class files rather than source files are the backing file type.

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Ancillary improvements to the spec.
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Appease jcheck.
 - Merge pull request #3 from lahodaj/JDK-8224922
   
   Improvements to JDK-8224922
 - Test cleanup.
 - Cleanup.
 - Cleanup.
 - Merge remote-tracking branch 'darcy/JDK-8224922' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - ... and 10 more: https://git.openjdk.java.net/jdk/compare/60d70e72...1599509b

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5038/files
  - new: https://git.openjdk.java.net/jdk/pull/5038/files/50ce61b2..1599509b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=06-07

  Stats: 1121 lines in 121 files changed: 632 ins; 121 del; 368 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5038.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5038/head:pull/5038

PR: https://git.openjdk.java.net/jdk/pull/5038