[lworld] Integrated: 8377617: [lworld] ProblemList compiler/stable/TestUnstableStable.java with enable-preview

[Daniel D. Daugherty]

A trivial fix to ProblemList compiler/stable/TestUnstableStable.java with enable-preview.

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Commit messages:
 - 8377617: [lworld] ProblemList compiler/stable/TestUnstableStable.java with enable-preview

Changes: https://git.openjdk.org/valhalla/pull/2076/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2076&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377617
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/2076.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2076/head:pull/2076

PR: https://git.openjdk.org/valhalla/pull/2076