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These tests were not functioning correctly because `assert_throws` was
not being passed an Error.  Furthermore, the use of the `expected.txt`
file is brittle since it includes a warning output with a line number.

BUG=769404

Change-Id: Ie867e6b99fc87d7c9b8b7259684f2ddd3cdc37a2
Reviewed-on: https://chromium-review.googlesource.com/688295Reviewed-by: Vincent Scheib <scheib@chromium.org>
Commit-Queue: Conley Owens <cco3@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505429}