RFR: JDK-8261623: reference to javac internals in Extern class [v2]

[Jonathan Gibbons]

> Trivial patch to move some code that accesses javac-internals into the `WorkArounds` class.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  add bug reference

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2537/files
  - new: https://git.openjdk.java.net/jdk/pull/2537/files/3ae30c81..708dfcb7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2537&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2537&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2537.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2537/head:pull/2537

PR: https://git.openjdk.java.net/jdk/pull/2537