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Also fix newly found JS Compiler errors (apparently ES6 classes trigger
stricter checks, which is good).

Bug: None
Change-Id: I22460e5e7c2fc6449317c31642ba1f69fe12eccc
Reviewed-on: https://chromium-review.googlesource.com/c/1279412
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599753}