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This can be used in lieu of mutating an ArrayList and using
Hamcrest matchers to check the behavior of Observables.

I have a couple of in-progress refactors that could use
something like this to help with testing.

Bug: None
Test: cast_base_junit_tests
Change-Id: Ieaac43cc2cb9d7f5b1577a7992b6211035fc36f9
Reviewed-on: https://chromium-review.googlesource.com/982514Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547462}