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

[Jonathan Gibbons]

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

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Commit messages:
 - JDK-8261623: reference to javac internals in Extern class

Changes: https://git.openjdk.java.net/jdk/pull/2537/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2537&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261623
  Stats: 24 lines in 2 files changed: 11 ins; 12 del; 1 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