RFR: 8224922: Access JavaFileObject from Element(s) [v5]

[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 six additional commits since the last revision:

 - Merge branch 'master' into JDK-8224922
 - Update spec.
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - 8224922: Access JavaFileObject from Element(s)

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5038/files
  - new: https://git.openjdk.java.net/jdk/pull/5038/files/9cdb6c93..d1b6fab1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=03-04

  Stats: 863 lines in 29 files changed: 673 ins; 124 del; 66 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