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This is needed to allow JS tests themselves to be compiled to ease
future maintenance. js2gtest.js also needs to be compiled for that,
which will be done in a follow-up.

Hand-crafted externs are added for mock4js since it is not able to
be consumed by the compiler directly.

axs_testing.js is precompiled and also not easily consumed by the
compiler, so appropriate suppressions are added to the a11y methods
of test_api.js that deal with it.

Bug: 1000989
Change-Id: Ife6b7595674ffc79c2eadc5484d0804830a648ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787477
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713357}