RFR: 8256152: tests fail because of ambiguous method resolution

[Stuart Marks]

Added a cast in the right place, thanks to @jonathan-gibbons.

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Commit messages:
 - 8256152: tests fail because of ambiguous method resolution

Changes: https://git.openjdk.java.net/jdk/pull/1274/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1274&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256152
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1274.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1274/head:pull/1274

PR: https://git.openjdk.java.net/jdk/pull/1274