HasConsecutiveItemsMatcher: Make it compile with JDK 9

Created by: davido

This problem is probably related to: [1].

Test Plan:

Apply this PR https://github.com/facebook/buck/pull/1047 and run:

buck build --config java.error_prone_javac=true
test/com/facebook/buck/testutil:testutil

[1] https://bugs.openjdk.java.net/browse/JDK-8039214