Integrated: JDK-8303540: Eliminate unnecessary reference to javac internal class

[Jonathan Gibbons]

Please review a trivial cleanup, to remove the unnecessary use of a javac internal type, when the public supertype is sufficient.

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Commit messages:
 - JDK-8303540:Eliminate unnecessary reference to javac internal class

Changes: https://git.openjdk.org/jdk/pull/12844/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12844&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303540
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12844.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12844/head:pull/12844

PR: https://git.openjdk.org/jdk/pull/12844