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This involves adding a conformance config to enforce the rule provided
by com.google.javascript.jscomp.ConformanceRules$BanNullDeref.

Fix the current non-conformance in release code. A follow-up will fix
the test code.

One instance in the Message.SAVE_COPY handler was likely a real bug.

Bug: 1068428, b/156695190
Change-Id: I80cb6b78b8b734cc272e636a1c680066af8ef0b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2203348
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: dstockwell <dstockwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769625}