RFR: 8261457: test/langtools/tools/javac/T8187978 can fail if ArrayList class is modified

[Adam Sotona]

Please review this simple test fix that removes coupling between this test and "live" library code by testing on provided nested classes as suggested in the bug comment.

Thank you,
Adam

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Commit messages:
 - 8261457: test/langtools/tools/javac/T8187978 can fail if ArrayList class is modified

Changes: https://git.openjdk.java.net/jdk/pull/2723/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2723&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261457
  Stats: 45 lines in 2 files changed: 41 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2723.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2723/head:pull/2723

PR: https://git.openjdk.java.net/jdk/pull/2723